Image decoding method for chroma component and device therefor

ABSTRACT

An image decoding method performed by a decoding device, according to the present document, comprises the steps of: deriving a chroma quantization parameter table on the basis of quantization parameter data; deriving a quantization parameter for combined chroma coding on the basis of the chroma quantization parameter table; deriving prediction samples of a current chroma block on the basis of prediction information; deriving transform coefficients of the current chroma block on the basis of residual information; deriving residual samples by dequantizing the transform coefficients on the basis of the quantization parameter; and generating a reconstructed picture on the basis of the prediction samples and the residual samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application PCT/KR2020/007559, with an internationalfiling date of Jun. 11, 2020, which claims the benefit of U.S.Provisional Patent Application No. 62/860,235, filed on Jun. 11, 2019,the contents of which are hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

This document relates to an image coding technology and, moreparticularly, to an image decoding method of coding image informationbased on chroma quantization parameter data signaled through a highlevel syntax in an image coding system and an apparatus thereof.

Related Art

Recently, demand for high-resolution, high-quality images, such as HighDefinition (HD) images and Ultra High Definition (UHD) images, has beenincreasing in various fields. As the image data has high resolution andhigh quality, the amount of information or bits to be transmittedincreases relative to the legacy image data. Therefore, when image datais transmitted using a medium such as a conventional wired/wirelessbroadband line or image data is stored using an existing storage medium,the transmission cost and the storage cost thereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high-resolution and high-quality images

SUMMARY

A technical object of the present disclosure is to provide a method andapparatus for improving image coding efficiency.

Another object of this document is to provide a method and apparatus forimproving efficiency of data coding for quantization parameterderivation for a chroma component.

According to an embodiment of this document, there is provided an imagedecoding method performed by a decoding apparatus. The method includesobtaining image information including prediction information andresidual information for a current chroma block, and a flag representingwhether quantization parameter data for combined chroma coding ispresent, based on a chroma type, obtaining the quantization parameterdata for the combined chroma coding based on the flag, deriving a chromaquantization parameter table based on the quantization parameter data,deriving a quantization parameter for the combined chroma coding basedon the chroma quantization parameter table, deriving prediction samplesof the current chroma block based on the prediction information,deriving transform coefficients of the current chroma block based on theresidual information, deriving residual samples by dequantizing thetransform coefficients based on the quantization parameter, andgenerating a reconstructed picture based on the prediction samples andthe residual samples, wherein the quantization parameter data includes asyntax element representing a starting index of the chroma quantizationparameter table and a syntax element representing a difference betweenthe starting index and a last index.

According to another embodiment of this document, there is provided adecoding apparatus performing image decoding. The decoding apparatusincludes an entropy decoder configured to obtain image informationincluding prediction information and residual information for a currentchroma block and a flag representing whether quantization parameter datafor combined chroma coding is present, based on a chroma type and toobtain the quantization parameter data for the combined chroma codingbased on the flag, a predictor configured to derive prediction samplesof the current chroma block based on the prediction information, aresidual processor configured to derive a chroma quantization parametertable based on the quantization parameter data, to derive a quantizationparameter for the combined chroma coding based on the chromaquantization parameter table, to derive transform coefficients of thecurrent chroma block based on the residual information, to deriveresidual samples by dequantizing the transform coefficients based on thequantization parameter, and an adder configured to generate areconstructed picture based on the prediction samples and the residualsamples, wherein the quantization parameter data includes a syntaxelement representing a starting index of the chroma quantizationparameter table and a syntax element representing a difference betweenthe starting index and a last index.

According to still another embodiment of this document, there isprovided a video encoding method performed by an encoding apparatus. Themethod includes deriving prediction samples of a current chroma blockbased on inter prediction or intra prediction, deriving residual samplesof the current chroma block based on the prediction samples, generatingquantization parameter data for combined chroma coding of the residualsamples based on a chroma type of the current chroma block, generating aflag representing whether the quantization parameter data for thecombined chroma coding is present, and encoding image informationincluding prediction information and residual information for thecurrent chroma block, the quantization parameter data and the flag,wherein the quantization parameter data includes a syntax elementrepresenting a starting index of a quantization parameter table for thecombined chroma coding and a syntax element representing a differencebetween the starting index and last index of the quantization parametertable.

According to still another embodiment of this document, there isprovided a video encoding apparatus. The encoding apparatus includes apredictor configured to derive prediction samples of a current chromablock based on inter prediction or intra prediction, a subtractorconfigured to derive residual samples of the current chroma block basedon the prediction samples, a residual processor configured to generatequantization parameter data for combined chroma coding of the residualsamples based on a chroma type of the current chroma block and togenerate a flag representing whether the quantization parameter data forthe combined chroma coding is present, and an entropy encoder configuredto encode image information including prediction information andresidual information for the current chroma block, the quantizationparameter data and the flag, wherein the quantization parameter dataincludes a syntax element representing a starting index of aquantization parameter table for the combined chroma coding and a syntaxelement representing a difference between the starting index and lastindex of the quantization parameter table.

Advantageous Effects

According to the present disclosure, a quantization parameter table forquantization parameter derivation can be determined based on a flagrepresenting whether quantization parameter data for deriving aquantization parameter for a chroma component has been transmitted, andcoding efficiency can be improved by performing coding based on aquantization parameter according to characteristics of an image.

According to the present disclosure, a quantization parameter table fora chroma component can be determined based on signaled chromaquantization data, and coding efficiency can be improved by performingcoding based on a quantization parameter according to characteristics ofan image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 4 illustrates an example of an inter prediction-based video/imageencoding method.

FIG. 5 illustrates an example of an inter prediction-based video/imagedecoding method.

FIG. 6 schematically shows an inter prediction procedure.

FIG. 7 schematically shows an image encoding method by an encodingapparatus according to the present document.

FIG. 8 schematically shows an encoding apparatus for performing an imageencoding method according to this document.

FIG. 9 schematically shows an image decoding method by a decodingapparatus according to this document.

FIG. 10 schematically shows a decoding apparatus for performing an imagedecoding method according to this document.

FIG. 11 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present disclosure may be modified in various forms, and specificembodiments thereof will be described and illustrated in the drawings.However, the embodiments are not intended for limiting the disclosure.The terms used in the following description are used to merely describespecific embodiments but are not intended to limit the disclosure. Anexpression of a singular number includes an expression of the pluralnumber, so long as it is clearly read differently. The terms such as“include” and “have” are intended to indicate that features, numbers,steps, operations, elements, components, or combinations thereof used inthe following description exist and it should be thus understood thatthe possibility of existence or addition of one or more differentfeatures, numbers, steps, operations, elements, components, orcombinations thereof is not excluded.

Meanwhile, elements in the drawings described in the disclosure areindependently drawn for the purpose of convenience for explanation ofdifferent specific functions, and do not mean that the elements areembodied by independent hardware or independent software. For example,two or more elements of the elements may be combined to form a singleelement, or one element may be partitioned into plural elements. Theembodiments in which the elements are combined and/or partitioned belongto the disclosure without departing from the concept of the disclosure.

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

Referring to FIG. 1, a video/image coding system may include a firstdevice (source device) and a second device (receiving device). Thesource device may deliver encoded video/image information or data in theform of a file or streaming to the receiving device via a digitalstorage medium or network.

The source device may include a video source, an encoding apparatus, anda transmitter. The receiving device may include a receiver, a decodingapparatus, and a renderer. The encoding apparatus may be called avideo/image encoding apparatus, and the decoding apparatus may be calleda video/image decoding apparatus. The transmitter may be included in theencoding apparatus. The receiver may be included in the decodingapparatus. The renderer may include a display, and the display may beconfigured as a separate device or an external component.

The video source may acquire video/image through a process of capturing,synthesizing, or generating the video/image. The video source mayinclude a video/image capture device and/or a video/image generatingdevice. The video/image capture device may include, for example, one ormore cameras, video/image archives including previously capturedvideo/images, and the like. The video/image generating device mayinclude, for example, computers, tablets and smartphones, and may(electronically) generate video/images. For example, a virtualvideo/image may be generated through a computer or the like. In thiscase, the video/image capturing process may be replaced by a process ofgenerating related data.

The encoding apparatus may encode input image/image. The encodingapparatus may perform a series of procedures such as prediction,transform, and quantization for compression and coding efficiency. Theencoded data (encoded video/image information) may be output in the formof a bit stream.

The transmitter may transmit the encoded image/image information or dataoutput in the form of a bit stream to the receiver of the receivingdevice through a digital storage medium or a network in the form of afile or streaming. The digital storage medium may include variousstorage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and thelike. The transmitter may include an element for generating a media filethrough a predetermined file format and may include an element fortransmission through a broadcast/communication network. The receiver mayreceive/extract the bit stream and transmit the received bit stream tothe decoding apparatus.

The decoding apparatus may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding apparatus.

The renderer may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Present disclosure relates to video/image coding. For example, themethods/embodiments disclosed in the present disclosure may be appliedto a method disclosed in the versatile video coding (VVC), the EVC(essential video coding) standard, the AOMedia Video 1 (AV1) standard,the 2nd generation of audio video coding standard (AVS2), or the nextgeneration video/image coding standard (e.g., H.267 or H.268, etc.).

Present disclosure presents various embodiments of video/image coding,and the embodiments may be performed in combination with each otherunless otherwise mentioned.

In the present disclosure, video may refer to a series of images overtime. Picture generally refers to a unit representing one image in aspecific time zone, and a subpicture/slice/tile is a unit constitutingpart of a picture in coding. The subpicture/slice/tile may include oneor more coding tree units (CTUs). One picture may consist of one or moresubpictures/slices/tiles. One picture may consist of one or more tilegroups. One tile group may include one or more tiles. A brick mayrepresent a rectangular region of CTU rows within a tile in a picture. Atile may be partitioned into multiple bricks, each of which consistingof one or more CTU rows within the tile. A tile that is not partitionedinto multiple bricks may be also referred to as a brick. A brick scan isa specific sequential ordering of CTUs partitioning a picture in whichthe CTUs are ordered consecutively in CTU raster scan in a brick, brickswithin a tile are ordered consecutively in a raster scan of the bricksof the tile, and tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. In addition, a subpicture mayrepresent a rectangular region of one or more slices within a picture.That is, a subpicture contains one or more slices that collectivelycover a rectangular region of a picture. A tile is a rectangular regionof CTUs within a particular tile column and a particular tile row in apicture. The tile column is a rectangular region of CTUs having a heightequal to the height of the picture and a width specified by syntaxelements in the picture parameter set. The tile row is a rectangularregion of CTUs having a height specified by syntax elements in thepicture parameter set and a width equal to the width of the picture. Atile scan is a specific sequential ordering of CTUs partitioning apicture in which the CTUs are ordered consecutively in CTU raster scanin a tile whereas tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. A slice includes an integernumber of bricks of a picture that may be exclusively contained in asingle NAL unit. A slice may consist of either a number of completetiles or only a consecutive sequence of complete bricks of one tile.Tile groups and slices may be used interchangeably in the presentdisclosure. For example, in the present disclosure, a tile group/tilegroup header may be called a slice/slice header.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma (e.g., cb, cr) blocks. The unit may be used interchangeably withterms such as block or area in some cases. In a general case, an M×Nblock may include samples (or sample arrays) or a set (or array) oftransform coefficients of M columns and N rows.

In the present description, “A or B” may mean “only A”, “only B” or“both A and B”. In other words, in the present specification, “A or B”may be interpreted as “A and/or B”. For example, “A, B or C” hereinmeans “only A”, “only B”, “only C”, or “any and any combination of A, Band C”.

A slash (/) or a comma (comma) used in the present description may mean“and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B”may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C”may mean “A, B, or C”.

In the present description, “at least one of A and B” may mean “only A”,“only B”, or “both A and B”. In addition, in the present description,the expression “at least one of A or B” or “at least one of A and/or B”may be interpreted the same as “at least one of A and B”.

In addition, in the present description, “at least one of A, B and C”means “only A”, “only B”, “only C”, or “any combination of A, B and C”.Also, “at least one of A, B or C” or “at least one of A, B and/or C” maymean “at least one of A, B and C”.

In addition, parentheses used in the present description may mean “forexample”. Specifically, when “prediction (intra prediction)” isindicated, “intra prediction” may be proposed as an example of“prediction”. In other words, “prediction” in the present description isnot limited to “intra prediction”, and “intra prediction” may beproposed as an example of “prediction”. Also, even when “prediction(i.e., intra prediction)” is indicated, “intra prediction” may beproposed as an example of “prediction”.

In the present description, technical features that are individuallydescribed within one drawing may be implemented individually or may beimplemented at the same time.

The following drawings were created to explain a specific example of thepresent description. Since the names of specific devices described inthe drawings or the names of specific signals/messages/fields arepresented by way of example, the technical features of the presentdescription are not limited to the specific names used in the followingdrawings.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied. Hereinafter, the video encoding apparatus mayinclude an image encoding apparatus.

Referring to FIG. 2, the encoding apparatus 200 includes an imagepartitioner 210, a predictor 220, a residual processor 230, and anentropy encoder 240, an adder 250, a filter 260, and a memory 270. Thepredictor 220 may include an inter predictor 221 and an intra predictor222. The residual processor 230 may include a transformer 232, aquantizer 233, a dequantizer 234, and an inverse transformer 235. Theresidual processor 230 may further include a subtractor 231. The adder250 may be called a reconstructor or a reconstructed block generator.The image partitioner 210, the predictor 220, the residual processor230, the entropy encoder 240, the adder 250, and the filter 260 may beconfigured by at least one hardware component (e.g., an encoder chipsetor processor) according to an embodiment. In addition, the memory 270may include a decoded picture buffer (DPB) or may be configured by adigital storage medium. The hardware component may further include thememory 270 as an internal/external component.

The image partitioner 210 may partition an input image (or a picture ora frame) input to the encoding apparatus 200 into one or moreprocessors. For example, the processor may be called a coding unit (CU).In this case, the coding unit may be recursively partitioned accordingto a quad-tree binary-tree ternary-tree (QTBTTT) structure from a codingtree unit (CTU) or a largest coding unit (LCU). For example, one codingunit may be partitioned into a plurality of coding units of a deeperdepth based on a quad tree structure, a binary tree structure, and/or aternary structure. In this case, for example, the quad tree structuremay be applied first and the binary tree structure and/or ternarystructure may be applied later. Alternatively, the binary tree structuremay be applied first. The coding procedure according to the presentdisclosure may be performed based on the final coding unit that is nolonger partitioned. In this case, the largest coding unit may be used asthe final coding unit based on coding efficiency according to imagecharacteristics, or if necessary, the coding unit may be recursivelypartitioned into coding units of deeper depth and a coding unit havingan optimal size may be used as the final coding unit. Here, the codingprocedure may include a procedure of prediction, transform, andreconstruction, which will be described later. As another example, theprocessor may further include a prediction unit (PU) or a transform unit(TU). In this case, the prediction unit and the transform unit may besplit or partitioned from the aforementioned final coding unit. Theprediction unit may be a unit of sample prediction, and the transformunit may be a unit for deriving a transform coefficient and/or a unitfor deriving a residual signal from the transform coefficient.

The unit may be used interchangeably with terms such as block or area insome cases. In a general case, an M×N block may represent a set ofsamples or transform coefficients composed of M columns and N rows. Asample may generally represent a pixel or a value of a pixel, mayrepresent only a pixel/pixel value of a luma component or represent onlya pixel/pixel value of a chroma component. A sample may be used as aterm corresponding to one picture (or image) for a pixel or a pel.

In the encoding apparatus 200, a prediction signal (predicted block,prediction sample array) output from the inter predictor 221 or theintra predictor 222 is subtracted from an input image signal (originalblock, original sample array) to generate a residual signal residualblock, residual sample array), and the generated residual signal istransmitted to the transformer 232. In this case, as shown, a unit forsubtracting a prediction signal (predicted block, prediction samplearray) from the input image signal (original block, original samplearray) in the encoder 200 may be called a subtractor 231. The predictormay perform prediction on a block to be processed (hereinafter, referredto as a current block) and generate a predicted block includingprediction samples for the current block. The predictor may determinewhether intra prediction or inter prediction is applied on a currentblock or CU basis. As described later in the description of eachprediction mode, the predictor may generate various information relatedto prediction, such as prediction mode information, and transmit thegenerated information to the entropy encoder 240. The information on theprediction may be encoded in the entropy encoder 240 and output in theform of a bit stream.

The intra predictor 222 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The non-directional mode may include, for example,a DC mode and a planar mode. The directional mode may include, forexample, 33 directional prediction modes or 65 directional predictionmodes according to the degree of detail of the prediction direction.However, this is merely an example, more or less directional predictionmodes may be used depending on a setting. The intra predictor 222 maydetermine the prediction mode applied to the current block by using aprediction mode applied to a neighboring block.

The inter predictor 221 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. Here, in order to reduce theamount of motion information transmitted in the inter prediction mode,the motion information may be predicted in units of blocks, sub-blocks,or samples based on correlation of motion information between theneighboring block and the current block. The motion information mayinclude a motion vector and a reference picture index. The motioninformation may further include inter prediction direction (L0prediction, L1 prediction, Bi prediction, etc.) information. In the caseof inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. The referencepicture including the reference block and the reference pictureincluding the temporal neighboring block may be the same or different.The temporal neighboring block may be called a collocated referenceblock, a co-located CU (colCU), and the like, and the reference pictureincluding the temporal neighboring block may be called a collocatedpicture (colPic). For example, the inter predictor 221 may configure amotion information candidate list based on neighboring blocks andgenerate information indicating which candidate is used to derive amotion vector and/or a reference picture index of the current block.Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the interpredictor 221 may use motion information of the neighboring block asmotion information of the current block. In the skip mode, unlike themerge mode, the residual signal may not be transmitted. In the case ofthe motion vector prediction (MVP) mode, the motion vector of theneighboring block may be used as a motion vector predictor and themotion vector of the current block may be indicated by signaling amotion vector difference.

The predictor 220 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply both intra prediction and inter prediction.This may be called combined inter and intra prediction (CIIP). Inaddition, the predictor may be based on an intra block copy (IBC)prediction mode or a palette mode for prediction of a block. The IBCprediction mode or palette mode may be used for content image/videocoding of a game or the like, for example, screen content coding (SCC).The IBC basically performs prediction in the current picture but may beperformed similarly to inter prediction in that a reference block isderived in the current picture. That is, the IBC may use at least one ofthe inter prediction techniques described in the present disclosure. Thepalette mode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The prediction signal generated by the predictor (including the interpredictor 221 and/or the intra predictor 222) may be used to generate areconstructed signal or to generate a residual signal. The transformer232 may generate transform coefficients by applying a transformtechnique to the residual signal. For example, the transform techniquemay include at least one of a discrete cosine transform (DCT), adiscrete sine transform (DST), a Karhunen-loeve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform generated based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 233 may quantize the transform coefficients and transmitthem to the entropy encoder 240 and the entropy encoder 240 may encodethe quantized signal (information on the quantized transformcoefficients) and output a bit stream. The information on the quantizedtransform coefficients may be referred to as residual information. Thequantizer 233 may rearrange block type quantized transform coefficientsinto a one-dimensional vector form based on a coefficient scanning orderand generate information on the quantized transform coefficients basedon the quantized transform coefficients in the one-dimensional vectorform. Information on transform coefficients may be generated. Theentropy encoder 240 may perform various encoding methods such as, forexample, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 240 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(e.g., values of syntax elements, etc.) together or separately. Encodedinformation (e.g., encoded video/image information) may be transmittedor stored in units of NALs (network abstraction layer) in the form of abit stream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. In the presentdisclosure, information and/or syntax elements transmitted/signaled fromthe encoding apparatus to the decoding apparatus may be included invideo/picture information. The video/image information may be encodedthrough the above-described encoding procedure and included in the bitstream. The bit stream may be transmitted over a network or may bestored in a digital storage medium. The network may include abroadcasting network and/or a communication network, and the digitalstorage medium may include various storage media such as USB, SD, CD,DVD, Blu-ray, HDD, SSD, and the like. A transmitter (not shown)transmitting a signal output from the entropy encoder 240 and/or astorage unit (not shown) storing the signal may be included asinternal/external element of the encoding apparatus 200, andalternatively, the transmitter may be included in the entropy encoder240.

The quantized transform coefficients output from the quantizer 233 maybe used to generate a prediction signal. For example, the residualsignal (residual block or residual samples) may be reconstructed byapplying dequantization and inverse transform to the quantized transformcoefficients through the dequantizer 234 and the inverse transformer235. The adder 250 adds the reconstructed residual signal to theprediction signal output from the inter predictor 221 or the intrapredictor 222 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 250 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied duringpicture encoding and/or reconstruction.

The filter 260 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter260 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 270, specifically, a DPB of thememory 270. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 260 may generate variousinformation related to the filtering and transmit the generatedinformation to the entropy encoder 240 as described later in thedescription of each filtering method. The information related to thefiltering may be encoded by the entropy encoder 240 and output in theform of a bit stream.

The modified reconstructed picture transmitted to the memory 270 may beused as the reference picture in the inter predictor 221. When the interprediction is applied through the encoding apparatus, predictionmismatch between the encoding apparatus 200 and the decoding apparatus300 may be avoided and encoding efficiency may be improved.

The DPB of the memory 270 DPB may store the modified reconstructedpicture for use as a reference picture in the inter predictor 221. Thememory 270 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 221 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 270 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra predictor 222.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

Referring to FIG. 3, the decoding apparatus 300 may include an entropydecoder 310, a residual processor 320, a predictor 330, an adder 340, afilter 350, and a memory 360. The predictor 330 may include an interpredictor 331 and an intra predictor 332. The residual processor 320 mayinclude a dequantizer 321 and an inverse transformer 322. The entropydecoder 310, the residual processor 320, the predictor 330, the adder340, and the filter 350 may be configured by a hardware component (e.g.,a decoder chipset or a processor) according to an embodiment. Inaddition, the memory 360 may include a decoded picture buffer (DPB) ormay be configured by a digital storage medium. The hardware componentmay further include the memory 360 as an internal/external component.

When a bit stream including video/image information is input, thedecoding apparatus 300 may reconstruct an image corresponding to aprocess in which the video/image information is processed in theencoding apparatus of FIG. 2. For example, the decoding apparatus 300may derive units/blocks based on block partition related informationobtained from the bit stream. The decoding apparatus 300 may performdecoding using a processor applied in the encoding apparatus. Thus, theprocessor of decoding may be a coding unit, for example, and the codingunit may be partitioned according to a quad tree structure, binary treestructure and/or ternary tree structure from the coding tree unit or thelargest coding unit. One or more transform units may be derived from thecoding unit. The reconstructed image signal decoded and output throughthe decoding apparatus 300 may be reproduced through a reproducingapparatus.

The decoding apparatus 300 may receive a signal output from the encodingapparatus of FIG. 2 in the form of a bit stream, and the received signalmay be decoded through the entropy decoder 310. For example, the entropydecoder 310 may parse the bit stream to derive information (e.g.,video/image information) necessary for image reconstruction (or picturereconstruction). The video/image information may further includeinformation on various parameter sets such as an adaptation parameterset (APS), a picture parameter set (PPS), a sequence parameter set(SPS), or a video parameter set (VPS). In addition, the video/imageinformation may further include general constraint information. Thedecoding apparatus may further decode picture based on the informationon the parameter set and/or the general constraint information.Signaled/received information and/or syntax elements described later inthe present disclosure may be decoded may decode the decoding procedureand obtained from the bit stream. For example, the entropy decoder 310decodes the information in the bit stream based on a coding method suchas exponential Golomb coding, CAVLC, or CABAC, and output syntaxelements required for image reconstruction and quantized values oftransform coefficients for residual. More specifically, the CABACentropy decoding method may receive a bin corresponding to each syntaxelement in the bit stream, determine a context model using a decodingtarget syntax element information, decoding information of a decodingtarget block or information of a symbol/bin decoded in a previous stage,and perform an arithmetic decoding on the bin by predicting aprobability of occurrence of a bin according to the determined contextmodel, and generate a symbol corresponding to the value of each syntaxelement. In this case, the CABAC entropy decoding method may update thecontext model by using the information of the decoded symbol/bin for acontext model of a next symbol/bin after determining the context model.The information related to the prediction among the information decodedby the entropy decoder 310 may be provided to the predictor (the interpredictor 332 and the intra predictor 331), and the residual value onwhich the entropy decoding was performed in the entropy decoder 310,that is, the quantized transform coefficients and related parameterinformation, may be input to the residual processor 320. The residualprocessor 320 may derive the residual signal (the residual block, theresidual samples, the residual sample array). In addition, informationon filtering among information decoded by the entropy decoder 310 may beprovided to the filter 350. Meanwhile, a receiver (not shown) forreceiving a signal output from the encoding apparatus may be furtherconfigured as an internal/external element of the decoding apparatus300, or the receiver may be a component of the entropy decoder 310.Meanwhile, the decoding apparatus according to the present disclosuremay be referred to as a video/image/picture decoding apparatus, and thedecoding apparatus may be classified into an information decoder(video/image/picture information decoder) and a sample decoder(video/image/picture sample decoder). The information decoder mayinclude the entropy decoder 310, and the sample decoder may include atleast one of the dequantizer 321, the inverse transformer 322, the adder340, the filter 350, the memory 360, the inter predictor 332, and theintra predictor 331.

The dequantizer 321 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 321 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock form. In this case, the rearrangement may be performed based onthe coefficient scanning order performed in the encoding apparatus. Thedequantizer 321 may perform dequantization on the quantized transformcoefficients by using a quantization parameter (e.g., quantization stepsize information) and obtain transform coefficients.

The inverse transformer 322 inversely transforms the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The predictor may perform prediction on the current block and generate apredicted block including prediction samples for the current block. Thepredictor may determine whether intra prediction or inter prediction isapplied to the current block based on the information on the predictionoutput from the entropy decoder 310 and may determine a specificintra/inter prediction mode.

The predictor 320 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply intra prediction and inter prediction. Thismay be called combined inter and intra prediction (CIIP). In addition,the predictor may be based on an intra block copy (IBC) prediction modeor a palette mode for prediction of a block. The IBC prediction mode orpalette mode may be used for content image/video coding of a game or thelike, for example, screen content coding (SCC). The IBC basicallyperforms prediction in the current picture but may be performedsimilarly to inter prediction in that a reference block is derived inthe current picture. That is, the IBC may use at least one of the interprediction techniques described in the present disclosure. The palettemode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The intra predictor 331 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The intra predictor 331 may determine theprediction mode applied to the current block by using a prediction modeapplied to a neighboring block.

The inter predictor 332 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. In this case, in order to reducethe amount of motion information transmitted in the inter predictionmode, motion information may be predicted in units of blocks,sub-blocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter predictor 332 may configure a motion information candidate listbased on neighboring blocks and derive a motion vector of the currentblock and/or a reference picture index based on the received candidateselection information. Inter prediction may be performed based onvarious prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 340 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the predictor (including the interpredictor 332 and/or the intra predictor 331). If there is no residualfor the block to be processed, such as when the skip mode is applied,the predicted block may be used as the reconstructed block.

The adder 340 may be called reconstructor or a reconstructed blockgenerator. The generated reconstructed signal may be used for intraprediction of a next block to be processed in the current picture, maybe output through filtering as described below, or may be used for interprediction of a next picture.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied in thepicture decoding process.

The filter 350 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter350 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 360, specifically, a DPB of thememory 360. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 360may be used as a reference picture in the inter predictor 332. Thememory 360 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 260 so as to be utilized as the motion informationof the spatial neighboring block or the motion information of thetemporal neighboring block. The memory 360 may store reconstructedsamples of reconstructed blocks in the current picture and transfer thereconstructed samples to the intra predictor 331.

In the present disclosure, the embodiments described in the filter 260,the inter predictor 221, and the intra predictor 222 of the encodingapparatus 200 may be the same as or respectively applied to correspondto the filter 350, the inter predictor 332, and the intra predictor 331of the decoding apparatus 300. The same may also apply to the unit 332and the intra predictor 331.

In the present disclosure, at least one of quantization/inversequantization and/or transform/inverse transform may be omitted. When thequantization/inverse quantization is omitted, the quantized transformcoefficients may be called transform coefficients. When thetransform/inverse transform is omitted, the transform coefficients maybe called coefficients or residual coefficients, or may still be calledtransform coefficients for uniformity of expression.

In the present disclosure, a quantized transform coefficient and atransform coefficient may be referred to as a transform coefficient anda scaled transform coefficient, respectively. In this case, the residualinformation may include information on transform coefficient(s), and theinformation on the transform coefficient(s) may be signaled throughresidual coding syntax. Transform coefficients may be derived based onthe residual information (or the information on the transformcoefficient(s)), and scaled transform coefficients may be derived byinverse transforming (scaling) on the transform coefficients. Residualsamples may be derived based on the inverse transforming (transforming)on the scaled transform coefficients. This may be applied/expressed inother parts of the present disclosure as well.

As described above, in performing video coding, prediction is performedto improve compression efficiency. Through this, a predicted blockincluding prediction samples for a current block as a block to be coded(i.e., a coding target block) may be generated. Here, the predictedblock includes prediction samples in a spatial domain (or pixel domain).The predicted block is derived in the same manner in an encodingapparatus and a decoding apparatus, and the encoding apparatus maysignal information (residual information) on residual between theoriginal block and the predicted block, rather than an original samplevalue of an original block, to the decoding apparatus, therebyincreasing image coding efficiency. The decoding apparatus may derive aresidual block including residual samples based on the residualinformation, add the residual block and the predicted block to generatereconstructed blocks including reconstructed samples, and generate areconstructed picture including the reconstructed blocks.

The residual information may be generated through a transform andquantization procedure. For example, the encoding apparatus may derive aresidual block between the original block and the predicted block,perform a transform procedure on residual samples (residual samplearray) included in the residual block to derive transform coefficients,perform a quantization procedure on the transform coefficients to derivequantized transform coefficients, and signal related residualinformation to the decoding apparatus (through a bit stream). Here, theresidual information may include value information of the quantizedtransform coefficients, location information, a transform technique, atransform kernel, a quantization parameter, and the like. The decodingapparatus may perform dequantization/inverse transform procedure basedon the residual information and derive residual samples (or residualblocks). The decoding apparatus may generate a reconstructed picturebased on the predicted block and the residual block. Also, for referencefor inter prediction of a picture afterward, the encoding apparatus mayalso dequantize/inverse-transform the quantized transform coefficientsto derive a residual block and generate a reconstructed picture basedthereon.

Intra prediction may refer to prediction that generates predictionsamples for a current block based on reference samples in a picture towhich the current block belongs (hereinafter, referred to as a currentpicture). When the intra prediction is applied to the current block,neighboring reference samples to be used for the intra prediction of thecurrent block may be derived. The neighboring reference samples of thecurrent block may include a sample adjacent to the left boundary of thecurrent block of size nW×nH and a total of 2×nH samples adjacent to thebottom-left of the current block, a sample adjacent to the top boundaryof the current block and a total of 2×nW samples adjacent to thetop-right and a sample adjacent to the top-left of the current block.Alternatively, the neighboring reference samples of the current blockmay include a plurality of columns of top neighboring samples and aplurality of rows of left neighboring samples. In addition, theneighboring reference samples of the current block may include a totalof nH samples adjacent to the right boundary of the current block ofsize nW×nH, a total of nW samples adjacent to the bottom boundary of thecurrent block and a sample adjacent to the bottom-right of the currentblock.

However, some of the neighboring reference samples of the current blockhave not yet been decoded or may not be available. In this case, thedecoder may construct neighboring reference samples to be used forprediction by substituting unavailable samples with available samples.Alternatively, neighboring reference samples to be used for predictionmay be configured through interpolation of available samples.

When the neighboring reference samples are derived, (i) a predictionsample may be derived based on the average or interpolation ofneighboring reference samples of the current block, or (ii) theprediction sample may be derived based on a reference sample existing ina specific (prediction) direction with respect to a prediction sampleamong the neighboring reference samples of the current block. The caseof (i) may be called a non-directional mode or a non-angular mode, andthe case of (ii) may be called a directional mode or an angular mode.

In addition, the prediction sample may be generated throughinterpolation of a first neighboring sample located in the predictiondirection of the intra prediction mode of the current block based on theprediction sample of the current block and a second neighboring samplelocated in a direction opposite to the prediction direction among theneighboring reference samples. The above-described case may be referredto as linear interpolation intra prediction (LIP). In addition, chromaprediction samples may be generated based on the luma samples using alinear model (LM). This case may be called an LM mode or a chromacomponent LM (CCLM) mode.

In addition, a temporary prediction sample of the current block isderived based on the filtered neighboring reference samples, and aprediction sample of the current block may also be derived by weightedsumming the temporary prediction sample and at least one referencesample derived according to the intra prediction mode among the existingneighboring reference samples, that is, unfiltered neighboring referencesamples. The above-described case may be referred to as positiondependent intra prediction (PDPC).

In addition, a reference sample line with the highest predictionaccuracy among neighboring multiple reference sample lines of thecurrent block is selected, and a prediction sample is derived using areference sample located in the prediction direction in the selectedline. In this case, intra prediction encoding may be performed byindicating (signaling) the used reference sample line to the decodingapparatus. The above-described case may be referred to asmulti-reference line intra prediction or MRL-based intra prediction.

In addition, the current block is divided into vertical or horizontalsub-partitions and intra prediction is performed based on the same intraprediction mode, but neighboring reference samples may be derived andused in units of the sub-partitions. That is, in this case, the intraprediction mode for the current block is equally applied to thesub-partitions, but the intra prediction performance may be improved insome cases by deriving and using the neighboring reference samples inunits of the sub-partitions. This prediction method may be calledintra-prediction based on intra sub-partitions (ISP).

The above-described intra prediction methods may be called intraprediction types to be distinguished from the intra prediction mode. Theintra prediction types may be referred to by various terms such as intraprediction technique or additional intra prediction modes. For example,the intra prediction types (or additional intra prediction modes, etc.)may include at least one of the aforementioned LIP, PDPC, MRL, and ISP.A general intra prediction method excluding a specific intra predictiontype such as LIP, PDPC, MRL, and ISP may be referred to as a normalintra prediction type. The normal intra prediction type may be generallyapplied when the above specific intra prediction type is not applied,and prediction may be performed based on the above-described intraprediction mode. Meanwhile, if necessary, post-processing filtering maybe performed on the derived prediction sample.

Specifically, the intra prediction process may include an intraprediction mode/type determination step, neighboring reference samplesderivation step, and an intra prediction mode/type based predictionsample derivation step. In addition, if necessary, a post-filtering stepmay be performed on the derived prediction sample.

When intra prediction is applied, an intra prediction mode applied tothe current block may be determined using an intra prediction mode of aneighboring block. For example, the decoding device may select one ofmost probable mode (MPM) candidates in the MPM list derived based onadditional candidate modes and an intra prediction mode of theneighboring block (e.g., the left and/or top neighboring block) of thecurrent block, or select one of the remaining intra prediction modes notincluded in the MPM candidates (and planar mode) based on the remainingintra prediction mode information. The MPM list may be configured toinclude or not include the planner mode as a candidate. For example,when the MPM list includes a planner mode as a candidate, the MPM listmay have 6 candidates, and when the MPM list does not include a plannermode as a candidate, the MPM list may have 5 candidates. When the MPMlist does not include the planar mode as a candidate, a not planar flag(e.g., intra_luma not_planar_flag) representing whether the intraprediction mode of the current block is not the planar mode may besignaled. For example, the MPM flag may be signaled first, and the MPMindex and not planner flag may be signaled when the value of the MPMflag is 1. Also, the MPM index may be signaled when the value of the notplanner flag is 1. Here, the fact that the MPM list is configured not toinclude the planner mode as a candidate is that the planner mode isalways considered as MPM rather than that the planner mode is not MPM,thus, the flag (not planar flag) is signaled first to check whether itis the planar mode.

For example, whether the intra prediction mode applied to the currentblock is among the MPM candidates (and the planar mode) or the remainingmodes may be indicated based on the MPM flag (e.g.,intra_luma_mpm_flag). The MPM flag with a value of 1 may indicate thatthe intra prediction mode for the current block is within MPM candidates(and planar mode), and The MPM flag with a value of 0 may indicate thatthe intra prediction mode for the current block is not within MPMcandidates (and planar mode). The not planar flag (e.g., intra_lumanot_planar flag) with a value of 0 may indicate that the intraprediction mode for the current block is a planar mode, and the notplanar flag with a value of 1 may indicate that the intra predictionmode for the current block is not the planar mode. The MPM index may besignaled in the form of an mpm_idx or intra_luma_mpm_idx syntax element,and the remaining intra prediction mode information may be signaled inthe form of a rem_intra_luma_pred_mode or intra_luma_mpm_remaindersyntax element. For example, the remaining intra prediction modeinformation may indicate one of the remaining intra prediction modes notincluded in the MPM candidates (and planar mode) among all intraprediction modes by indexing in the order of prediction mode number. Theintra prediction mode may be an intra prediction mode for a lumacomponent (sample). Hereinafter, the intra prediction mode informationmay include at least one of the MPM flag (e.g., intra_luma_mpm_flag),the not planar flag (e.g., intra_luma not_planar_flag), the MPM index(e.g., mpm_idx or intra_luma_mpm_idx), or the remaining intra predictionmode information (rem_intra_luma_luma_mpm_mode or intra_luma_mpminder).In the present disclosure, the MPM list may be referred to by variousterms such as an MPM candidate list and candModeList. When the MIP isapplied to the current block, a separate MPM flag (e.g.,intra_mip_mpm_flag) for the MIP, an MPM index (e.g., intra_mip_mpm_idx),and remaining intra prediction mode information (e.g.,intra_mip_mpm_remainder) may be signaled, and the not planar flag maynot be signaled.

In other words, in general, when a block partition for an image isperformed, the current block to be coded and a neighboring block havesimilar image characteristics. Therefore, there is a high probabilitythat the current block and the neighboring block have the same orsimilar intra prediction mode. Accordingly, the encoder may use theintra prediction mode of the neighboring block to encode the intraprediction mode of the current block.

For example, The decoding device/encoding device may construct a mostprobable modes (MPM) list for the current block. The MPM list may bereferred to as the MPM candidate list. Here, the MPM may refer to modesused to improve coding efficiency in consideration of the similaritybetween the current block and the neighboring blocks during intraprediction mode coding. As described above, the MPM list may beconstructed to include the planar mode, or may be constructed to excludethe planar mode. For example, when the MPM list includes the planarmode, the number of candidates in the MPM list may be 6. And, when theMPM list does not include the planar mode, the number of candidates inthe MPM list may be 5.

The encoder/decoder may construct an MPM list including five or sixMPMs.

In order to construct the MPM list, three types of modes, such asdefault intra modes, neighbor intra modes, and derived intra modes, maybe considered.

For the neighbor intra modes, two neighbor blocks, that is, a leftneighbor block and a top neighbor block, may be considered.

As described above, if the MPM list is constructed to not include aplanar mode, the planar mode may be excluded from the list, and thenumber of MPM list candidates may be set to five.

Furthermore, a non-directional mode (or a non-angle mode) among theintra prediction modes may include a DC mode based on an average ofneighbor reference samples of a current block or an interpolation-basedplanar mode.

Meanwhile, when inter prediction is applied, the predictor of theencoding apparatus/decoding apparatus may derive prediction samples byperforming inter prediction in units of blocks. The inter prediction maybe applied when performing the prediction on the current block. That is,the predictor (more specifically, inter predictor) of theencoding/decoding apparatus may derive prediction samples by performingthe inter prediction in units of the block. The inter prediction mayrepresent prediction derived by a method dependent to the data elements(e.g., sample values or motion information) of a picture(s) other thanthe current picture. When the inter prediction is applied to the currentblock, a predicted block (prediction sample array) for the current blockmay be derived based on a reference block (reference sample array)specified by the motion vector on the reference picture indicated by thereference picture index. In this case, in order to reduce an amount ofmotion information transmitted in the inter-prediction mode, the motioninformation of the current block may be predicted in units of a block, asubblock, or a sample based on a correlation of the motion informationbetween the neighboring block and the current block. The motioninformation may include the motion vector and the reference pictureindex. The motion information may further include inter-prediction type(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of applying the inter prediction, the neighboring block may includea spatial neighboring block which is present in the current picture anda temporal neighboring block which is present in the reference picture.A reference picture including the reference block and a referencepicture including the temporal neighboring block may be the same as eachother or different from each other. The temporal neighboring block maybe referred to as a name such as a collocated reference block, acollocated CU (colCU), etc., and the reference picture including thetemporal neighboring block may be referred to as a collocated picture(colPic). For example, a motion information candidate list may beconfigured based on the neighboring blocks of the current block and aflag or index information indicating which candidate is selected (used)may be signaled in order to derive the motion vector and/or referencepicture index of the current block. The inter prediction may beperformed based on various prediction modes and for example, in the caseof a skip mode and a merge mode, the motion information of the currentblock may be the same as the motion information of the selectedneighboring block. In the case of the skip mode, the residual signal maynot be transmitted unlike the merge mode. In the case of a motion vectorprediction (MVP) mode, the motion vector of the selected neighboringblock may be used as a motion vector predictor and a motion vectordifference may be signaled. In this case, the motion vector of thecurrent block may be derived by using a sum of the motion vectorpredictor and the motion vector difference.

The motion information may further include L0 motion information and/orL1 motion information according to the inter-prediction type (L0prediction, L1 prediction, Bi prediction, etc.). A L0-direction motionvector may be referred to as an L0 motion vector or MVL0 and anL1-direction motion vector may be referred to as an L1 motion vector orMVL1. A prediction based on the L0 motion vector may be referred to asan L0 prediction, a prediction based on the L1 motion vector may bereferred to as an L1 prediction, and a prediction based on both the L0motion vector and the L1 motion vector may be referred to as abi-prediction. Here, the L0 motion vector may indicate a motion vectorassociated with a reference picture list L0 and the L1 motion vector mayindicate a motion vector associated with a reference picture list L1.The reference picture list L0 may include pictures prior to the currentpicture in an output order and the reference picture list L1 may includepictures subsequent to the current picture in the output order, as thereference pictures. The prior pictures may be referred to as a forward(reference) picture and the subsequent pictures may be referred to as areverse (reference) picture. The reference picture list L0 may furtherinclude the pictures subsequent to the current picture in the outputorder as the reference pictures. In this case, the prior pictures may befirst indexed in the reference picture list L0 and the subsequentpictures may then be indexed. The reference picture list L1 may furtherinclude the pictures prior to the current picture in the output order asthe reference pictures. In this case, the subsequent pictures may befirst indexed in the reference picture list L1 and the prior picturesmay then be indexed. Here, the output order may correspond to a pictureorder count (POC) order.

A video/image encoding process based on inter prediction mayschematically include, for example, the following.

FIG. 4 illustrates an example of an inter prediction-based video/imageencoding method.

The encoding apparatus performs the inter prediction for the currentblock (S400). The encoding apparatus may derive the inter predictionmode and the motion information of the current block and generate theprediction samples of the current block. Here, an inter prediction modedetermining process, a motion information deriving process, and ageneration process of the prediction samples may be simultaneouslyperformed and any one process may be performed earlier than otherprocess. For example, the inter-prediction unit of the encodingapparatus may include a prediction mode determination unit, a motioninformation derivation unit, and a prediction sample derivation unit,and the prediction mode determination unit may determine the predictionmode for the current block, the motion information derivation unit mayderive the motion information of the current block, and the predictionsample derivation unit may derive the prediction samples of the currentblock. For example, the inter-prediction unit of the encoding apparatusmay search a block similar to the current block in a predetermined area(search area) of reference pictures through motion estimation and derivea reference block in which a difference from the current block isminimum or is equal to or less than a predetermined criterion. Areference picture index indicating a reference picture at which thereference block is positioned may be derived based thereon and a motionvector may be derived based on a difference in location between thereference block and the current block. The encoding apparatus maydetermine a mode applied to the current block among various predictionmodes. The encoding apparatus may compare RD cost for the variousprediction modes and determine an optimal prediction mode for thecurrent block.

For example, when the skip mode or the merge mode is applied to thecurrent block, the encoding apparatus may configure a merging candidatelist to be described below and derive a reference block in which adifference from the current block is minimum or is equal to or less thana predetermined criterion among reference blocks indicated by mergecandidates included in the merging candidate list. In this case, a mergecandidate associated with the derived reference block may be selectedand merge index information indicating the selected merge candidate maybe generated and signaled to the decoding apparatus. The motioninformation of the current block may be derived by using the motioninformation of the selected merge candidate.

As another example, when an (A)MVP mode is applied to the current block,the encoding apparatus may configure an (A)MVP candidate list to bedescribed below and use a motion vector of a selected mvp candidateamong motion vector predictor (mvp) candidates included in the (A)MVPcandidate list as the mvp of the current block. In this case, forexample, the motion vector indicating the reference block derived by themotion estimation may be used as the motion vector of the current blockand an mvp candidate having a motion vector with a smallest differencefrom the motion vector of the current block among the mvp candidates maybecome the selected mvp candidate. A motion vector difference (MVD)which is a difference obtained by subtracting the mvp from the motionvector of the current block may be derived. In this case, theinformation on the MVD may be signaled to the decoding apparatus.Further, when the (A)MVP mode is applied, the value of the referencepicture index may be configured as reference picture index informationand separately signaled to the decoding apparatus.

The encoding apparatus may derive the residual samples based on thepredicted samples (S410). The encoding apparatus may derive the residualsamples by comparing original samples and the prediction samples of thecurrent block.

The encoding apparatus encodes image information including predictioninformation and residual information (S420). The encoding apparatus mayoutput the encoded image information in the form of a bit stream. Theprediction information may include information on prediction modeinformation (e.g., skip flag, merge flag or mode index, etc.) andinformation on motion information as information related to theprediction procedure. The information on the motion information mayinclude candidate selection information (e.g., merge index, mvp flag ormvp index) which is information for deriving the motion vector. Further,the information on the motion information may include the information onthe MVD and/or the reference picture index information. Further, theinformation on the motion information may include information indicatingwhether to apply the L0 prediction, the L1 prediction, or thebi-prediction. The residual information is information on the residualsamples. The residual information may include information on quantizedtransform coefficients for the residual samples.

An output bit stream may be stored in a (digital) storage medium andtransferred to the decoding apparatus or transferred to the decodingapparatus via the network.

Meanwhile, as described above, the encoding apparatus may generate areconstructed picture (including reconstructed samples and reconstructedblocks) based on the reference samples and the residual samples. This isto derive the same prediction result as that performed by the decodingapparatus, and as a result, coding efficiency may be increased.Accordingly, the encoding apparatus may store the reconstruction picture(or reconstruction samples or reconstruction blocks) in the memory andutilize the reconstruction picture as the reference picture. The in-loopfiltering process may be further applied to the reconstruction pictureas described above.

A video/image decoding process based on inter prediction mayschematically include, for example, the following.

FIG. 5 illustrates an example of an inter prediction-based video/imagedecoding method.

Referring to FIG. 5, the decoding apparatus may perform an operationcorresponding to the operation performed by the encoding apparatus. Thedecoding apparatus may perform the prediction for the current blockbased on received prediction information and derive the predictionsamples.

Specifically, the decoding apparatus may determine the prediction modefor the current block based on the received prediction information(S500). The decoding apparatus may determine which inter prediction modeis applied to the current block based on the prediction mode informationin the prediction information.

For example, it may be determined whether the merge mode or the (A)MVPmode is applied to the current block based on the merge flag.Alternatively, one of various inter prediction mode candidates may beselected based on the mode index. The inter prediction mode candidatesmay include a skip mode, a merge mode, and/or an (A)MVP mode or mayinclude various inter prediction modes to be described below.

The decoding apparatus derives the motion information of the currentblock based on the determined inter prediction mode (S510). For example,when the skip mode or the merge mode is applied to the current block,the decoding apparatus may configure the merge candidate list to bedescribed below and select one merge candidate among the mergecandidates included in the merge candidate list. Here, the selection maybe performed based on the selection information (merge index). Themotion information of the current block may be derived by using themotion information of the selected merge candidate. The motioninformation of the selected merge candidate may be used as the motioninformation of the current block.

As another example, when an (A)MVP mode is applied to the current block,the decoding apparatus may configure an (A)MVP candidate list to bedescribed below and use a motion vector of a selected mvp candidateamong motion vector predictor (mvp) candidates included in the (A)MVPcandidate list as the mvp of the current block. Here, the selection maybe performed based on the selection information (mvp flag or mvp index).In this case, the MVD of the current block may be derived based on theinformation on the MVD, and the motion vector of the current block maybe derived based on the mvp of the current block and the MVD. Further,the reference picture index of the current block may be derived based onthe reference picture index information. The picture indicated by thereference picture index in the reference picture list for the currentblock may be derived as the reference picture referred for the interprediction of the current block.

Meanwhile, as described below, the motion information of the currentblock may be derived without a candidate list configuration and in thiscase, the motion information of the current block may be derivedaccording to a procedure disclosed in the prediction mode. In this case,the candidate list configuration may be omitted.

The decoding apparatus may generate the prediction samples for thecurrent block based on the motion information of the current block(S520). In this case, the reference picture may be derived based on thereference picture index of the current block and the prediction samplesof the current block may be derived by using the samples of thereference block indicated by the motion vector of the current block onthe reference picture. In this case, in some cases, a predicted samplefiltering procedure for all or some of the prediction samples of thecurrent block may be further performed.

For example, the inter-prediction unit of the decoding apparatus mayinclude a prediction mode determination unit, a motion informationderivation unit, and a prediction sample derivation unit, and theprediction mode determination unit may determine the prediction mode forthe current block based on the received prediction mode information, themotion information derivation unit may derive the motion information(the motion vector and/or reference picture index) of the current blockbased on the information on the received motion information, and theprediction sample derivation unit may derive the predicted samples ofthe current block.

The decoding apparatus generates the residual samples for the currentblock based on the received residual information (S530). The decodingapparatus may generate the reconstruction samples for the current blockbased on the prediction samples and the residual samples and generatethe reconstruction picture based on the generated reconstruction samples(S540). Thereafter, the in-loop filtering procedure may be furtherapplied to the reconstruction picture as described above.

FIG. 6 schematically shows an inter prediction procedure.

Referring to FIG. 6, as described above, the inter prediction processmay include an inter prediction mode determination step, a motioninformation derivation step according to the determined prediction mode,and a prediction processing (prediction sample generation) step based onthe derived motion information. The inter prediction process may beperformed by the encoding apparatus and the decoding apparatus asdescribed above. In this document, a coding device may include theencoding apparatus and/or the decoding apparatus.

Referring to FIG. 6, the coding apparatus determines an inter predictionmode for the current block (S600). Various inter prediction modes may beused for the prediction of the current block in the picture. Forexample, various modes, such as a merge mode, a skip mode, a motionvector prediction (MVP) mode, an affine mode, a subblock merge mode, amerge with MVD (MMVD) mode, and a historical motion vector prediction(HMVP) mode may be used. A decoder side motion vector refinement (DMVR)mode, an adaptive motion vector resolution (AMVR) mode, a bi-predictionwith CU-level weight (BCW), a bi-directional optical flow (BDOF), andthe like may be further used as additional modes. The affine mode mayalso be referred to as an affine motion prediction mode. The MVP modemay also be referred to as an advanced motion vector prediction (AMVP)mode. In the present document, some modes and/or motion informationcandidates derived by some modes may also be included in one of motioninformation-related candidates in other modes. For example, the HMVPcandidate may be added to the merge candidate of the merge/skip modes,or also be added to an mvp candidate of the MVP mode. If the HMVPcandidate is used as the motion information candidate of the merge modeor the skip mode, the HMVP candidate may be referred to as the HMVPmerge candidate.

The prediction mode information indicating the inter prediction mode ofthe current block may be signaled from the encoding apparatus to thedecoding apparatus. In this case, the prediction mode information may beincluded in the bit stream and received by the decoding apparatus. Theprediction mode information may include index information indicating oneof multiple candidate modes. Alternatively, the inter prediction modemay be indicated through a hierarchical signaling of flag information.In this case, the prediction mode information may include one or moreflags. For example, whether to apply the skip mode may be indicated bysignaling a skip flag, whether to apply the merge mode may be indicatedby signaling a merge flag when the skip mode is not applied, and it isindicated that the MVP mode is applied or a flag for additionaldistinguishing may be further signaled when the merge mode is notapplied. The affine mode may be signaled as an independent mode orsignaled as a dependent mode on the merge mode or the MVP mode. Forexample, the affine mode may include an affine merge mode and an affineMVP mode.

The coding apparatus derives motion information for the current block(S610). Motion information derivation may be derived based on the interprediction mode.

The coding apparatus may perform inter prediction using motioninformation of the current block. The encoding apparatus may deriveoptimal motion information for the current block through a motionestimation procedure. For example, the encoding apparatus may search asimilar reference block having a high correlation in units of afractional pixel within a predetermined search range in the referencepicture by using an original block in an original picture for thecurrent block and derive the motion information through the searchedreference block. The similarity of the block may be derived based on adifference of phase based sample values. For example, the similarity ofthe block may be calculated based on a sum of absolute differences (SAD)between the current block (or a template of the current block) and thereference block (or the template of the reference block). In this case,the motion information may be derived based on a reference block havinga smallest SAD in a search area. The derived motion information may besignaled to the decoding apparatus according to various methods based onthe inter prediction mode.

The coding apparatus performs inter prediction based on motioninformation for the current block (S620). The coding apparatus mayderive prediction sample(s) for the current block based on the motioninformation. A current block including prediction samples may bereferred to as a predicted block.

Meanwhile, as described above, the quantizer of the encoding apparatusmay derive quantized transform coefficients by applying quantization totransform coefficients. The dequantizer of the encoding apparatus or thedequantizer of the decoding apparatus may derive transform coefficientsby applying dequantization to the quantized transform coefficients.

In general, in video/image coding, a quantization ratio may be changed,and a compression rate may be adjusted using the changed quantizationratio. In an implementation aspect, a quantization parameter (QP) may beused instead of directly using the quantization ratio by consideringcomplexity. For example, quantization parameters having integer valuesof 0 to 63 may be used, and each quantization parameter value maycorrespond to an actual quantization ratio. Furthermore, for example, aquantization parameter QP_(Y) for a luma component and a quantizationparameter QP_(C) for a chroma component may be different configured.

In a quantization process, a transform coefficient C may be an input, aquantization ratio (Q_(step)) may be divided, and a quantized transformcoefficient C′ may be obtained based on the quantization ratio. In thiscase, the quantization ratio may be produced in an integer form bymultiplying the quantization ratio by a scale by considering calculationcomplexity, and a shift operation may be performed by a valuecorresponding to a scale value. A quantization scale may be derivedbased on the product of the quantization ratio and the scale value. Thatis, the quantization scale may be derived based on the QP. For example,the quantization scale may be applied to the transform coefficient C′,and a quantized transform coefficient C′ may be derived based on aresult of the application.

A dequantization process is a reverse process of the quantizationprocess. In this process, a quantized transform coefficient C′ may bemultiplied by a quantization ratio (Q_(step)), and a reconstructedtransform coefficient C″ may be obtained based on the result of themultiplication. In this case, a level scale may be derived based on aquantization parameter, the level scale may be applied to the quantizedtransform coefficient C′, and a reconstructed transform coefficient C″may be derived. The reconstructed transform coefficient C″ may have somedifference from the first transform coefficient C due to a loss in thetransform and/or quantization process. Accordingly, dequantization isperformed in the encoding apparatus as in the decoding apparatus.

Meanwhile, an adaptive frequency weighting quantization technology foradjusting quantization strength depending on a frequency may be applied.The adaptive frequency weighting quantization technology is a method ofdifferently applying quantization strength for each frequency. In theadaptive frequency weighting quantization, quantization strength foreach frequency may be differently applied using a predefinedquantization scaling matrix. That is, the aforementionedquantization/dequantization process may be performed based on thequantization scaling matrix. For example, in order to generate the sizeof a current block and/or a residual signal of the current block, adifferent quantization scaling matrix may be used depending on whether aprediction mode applied to the current block is inter prediction orintra prediction. The quantization scaling matrix may be called aquantization matrix or a scaling matrix. The quantization scaling matrixmay be pre-defined. Furthermore, for frequency adaptive scaling,quantization scale information for each frequency for the quantizationscaling matrix may be constructed/encoded in the encoding apparatus andsignaled to the decoding apparatus. The quantization scale informationfor each frequency may be called quantization scaling information. Thequantization scale information for each frequency may include scalinglist data (scaling list_data). A (modified) quantization scaling matrixmay be derived based on the scaling list data. Furthermore, thequantization scale information for each frequency may include presentflag information representing whether the scaling list data is present.Alternatively, if the scaling list data is signaled in a higher level(e.g., SPS), information representing whether the scaling list data ismodified in a lower level (e.g., a PPS or a tile group header, etc.) ofa higher level, etc. may be further included.

As in the aforementioned contents, quantization/dequantization may beapplied to a luma component and a chroma component based on aquantization parameter.

Quantization parameters for a coding unit may be determined based oninformation signaled in a picture and/or a slice level. For example, thequantization parameters may be derived as in contents described later.

For example, information related to the derivation of quantizationparameters may be signaled as in the following table through a sequenceparameter set (SPS).

TABLE 1   seq_parameter_set_rbsp( ) { Descriptor   sps_decoding_parameter_set_id u(4)   ...    bit_depth_luma_minus8 ue(v)   bit_depth_chroma_minus8 ue(v)   ...    rbsp_trailing_bits( )   }

Semantics for syntax elements in Table 1 may be the same as thefollowing table.

TABLE 2 bit_depth_luma_minus8 specifies the bit depth of the samples ofthe luma array BitDepth_(Y) and the value of the luma quantizationparameter range offset QpBdOffset_(Y) as follows:  BitDepth_(Y) = 8 +bit_depth_luma_minus8  QpBdOffset_(Y) = 6 * bit_depth_luma_minus8bit_depth_luma_minus8 shall be in the range of 0 to 8, inclusive.bit_depth_chroma_minus8 specifies the bit depth of the samples of thechroma arrays BitDepth_(C) and the value of the chroma quantizationparameter range offset QpBdOffset_(C) as follows:  BitDepth_(C) = 8 + bit_depth_chroma_minus8  QpBdOffset_(C) = 6 * bit_depth_chroma_minus8bit_depth_chroma_minus8 shall be in the range of 0 to 8, inclusive.

For example, the syntax element bit_depth_luma_minus8 may representBitDepth_(Y), that is, the bit depth of samples of a luma array, andQpBdOffset_(Y) that is a luma quantization parameter range offset. Thatis, for example, the BitDepth_(Y) and the QpBdOffset_(Y) may be derivedbased on the syntax element bit_depth_luma_minus8. For example, theBitDepth_(Y) may be derived as a value obtained by adding 8 to a valueof the syntax element bit_depth_luma_minus8. The QpBdOffset_(Y) may bederived as a value obtained by multiplying a value of the syntax elementbit_depth_luma_minus8 by 6. Furthermore, the bit_depth_luma_minus8 maybe in a range of 0 to 8.

Furthermore, for example, the syntax element bit_depth_chroma_minus8 mayrepresent BitDepth_(c), that is, the bit depth of samples of a chromaarray, and QpBdOffset_(c), that is, a chroma quantization parameterrange offset. That is, for example, the BitDepth_(e) and theQpBdOffset_(c) may be derived based on the syntax elementbit_depth_chroma_minus8. For example, the BitDepth₀ may be derived as avalue obtained by adding 8 to a value of the syntax elementbit_depth_chroma_minus8. The QpBdOffset_(c) may be derived as a valueobtained by multiplying a value of the syntax elementbit_depth_chroma_minus8 by 6. Furthermore, the bit_depth_chroma_minus8may be in a range of 0 to 8.

Furthermore, information related to the derivation of quantizationparameters may be signaled as in the following table, for example,through a picture parameter set (PPS). The information may include achroma Cb offset, a chroma Cr offset, a joint chroma offset, and aninitial quantization parameter. That is, the information may includesyntax elements for a chroma Cb offset, a chroma Cr offset, a jointchroma offset, and an initial quantization parameter.

TABLE 3   pic_parameter_set_rbsp( ) { Descriptor   pps_pic_parameter_set_id ue(v)   ...    init_qp_minus26 se(v)   transform_skip_enabled_flag u(1)    pps_cb_qp_offset se(v)   pps_cr_qp_offset se(v)    pps_joint_cbcr_qp_offset se(v)   pps_slice_chroma_qp_offsets_present_flag u(1)   ...   rbsp_trailing_bits( )   }

Semantics for syntax elements in Table 3 may be the same as thefollowing table.

TABLE 4 init_qp_minus26 plus 26 specifies the initial value ofSliceQp_(Y) for each slice referring to the PPS. The initial value ofSliceQp_(Y) is modified at the slice layer when a non-zero value ofslice_qp_delta is decoded. The value of init_qp_minus26 shall be in therange of −( 26 + QpBdOffset_(Y) ) to +37, inclusive. pps_cb_qp_offsetand pps_cr_qp_offset specify the offsets to the luma quantizationparameter Qp′_(Y) used for deriving Qp′_(Cb) and Qp′_(Cr), respectively.The values of pps_cb_qp_offset and pps_cr_qp_offset shall be in therange of −12 to +12, inclusive. When ChromaArrayType is equal to 0,pps_cb_qp_offset and pps_cr_qp_offset are not used in the decodingprocess and decoders shall ignore their value. pps_joint_cbcr_qp_offsetspecifies the offset to the luma quantization parameter Qp′_(Y) used forderiving Qp′_(CbCr). The value of pps_joint_cbcr_qp_offset shall be inthe range of −12 to +12, inclusive. When ChromaArrayType is equal to 0,pps_joint_cbcr_qp_offset is not used in the decoding process anddecoders shall ignore its value.pps_slice_chroma_qp_offsets_present_flag equal to 1 indicates that theslice_cb_qp_offset and slice_cr_qp_offset syntax elements are present inthe associated slice headers. pps_slice_chroma_qp_offsets_present_flagequal to 0 indicates that these syntax elements are not present in theassociated slice headers. When ChromaArrayType is equal to 0,pps_slice_chroma_qp_offsets_present_flag shall be equal to 0.

For example, a value obtained by adding 26 to the syntax elementinit_qp_minus26 may represent an initial value of SliceQp_(Y) for eachslice that refers to a PPS. If a non-zero value of slice_qp_delta isdecoded, an initial value of the SliceQpY may be modified in a slicelayer. The init_qp_minus26 0 may be in a range of −(26+QpBdOffset_(Y))to +37.

Furthermore, for example, syntax elements pps_cb_qp_offset andpps_cr_qp_offset may represent offsets for a luma quantization parameterQp′_(Y) used to derive Qp′_(Cb) and Qp′_(Cr), respectively. Thepps_cb_qp_offset and pps_cr_qp_offset may be in a range of −12 to +12.Furthermore, when ChromaArrayType is 0, in a decoding process,pps_cb_qp_offset and pps_cr_qp_offset may not be used, and the decodingapparatus may ignore values of the syntax elements.

Furthermore, for example, the syntax element pps_joint_cbcr_qp_offsetmay represent an offset for a luma quantization parameter Qp′_(Y) usedto derive Qp′_(CbCr). The pps_joint_cbcr_qp_offset may be in a range of−12 to +12. Furthermore, when ChromaArrayType is 0, in a decodingprocess, pps_joint_cbcr_qp_offset may not be used, and the decodingapparatus may ignore a value of the syntax element.

Furthermore, for example, the syntax elementpps_slice_chroma_qp_offsets_present_flag may represent whether syntaxelements slice_cb_qp_offset and slice_cr_qp_offset are present in sliceheaders associated with the syntax elements slice_cb_qp_offset andslice_cr_qp_offset. For example,pps_slice_chroma_qp_offsets_present_flag having a value of 1 mayrepresent that the syntax elements slice_cb_qp_offset andslice_cr_qp_offset are present in slice headers associated with thesyntax elements slice_cb_qp_offset and slice_cr_qp_offset. Furthermore,for example, pps_slice_chroma_qp_offsets_present_flag having a value of0 may represent that the syntax elements slice_cb_qp_offset andslice_cr_qp_offset are not present in slice headers associated with thesyntax elements slice_cb_qp_offset and slice_cr_qp_offset. Furthermore,when ChromaArrayType is 0, in a decoding process,pps_slice_chroma_qp_offsets_present_flag may be the same as 0.

As in the aforementioned contents, syntax elements parsed in the PPS maybe init_qp_minus26, pps_cb_qp_offset_pps_cr_qp_offset,pps_joint_cbcr_qp_offset, and pps_slice_chroma_qp_offsets_present_flag.A syntax element init_qp_minus26 may represent an initial value ofSliceQp_(Y) for each slice that refers to a PPS. Furthermore, syntaxelements pps_cb_qp_offset, pps_cr_qp_offset, andpps_joint_cbcr_qp_offset may represent offsets for a luma quantizationparameter Qp′_(Y). Furthermore, the syntax elementpps_slice_chroma_qp_offsets_present_flag may represent whether an offsetparameter is present in a slice header.

Furthermore, information related to the derivation of quantizationparameters may be signaled as in the following table through a sliceheader, for example.

TABLE 5   slice_header( ) { Descriptor    slice_pic_parameter_set_idue(v) ...    slice_qp_delta se(v)    if(pps_slice_chroma_qp_offsets_present_flag ) {     slice_cb_qp_offsetse(v)     slice_cr_qp_offset se(v)     slice_joint_cbcr_qp_offset se(v)   }   ...    byte_alignment( )   }

Semantics for syntax elements in Table 5 may be the same as thefollowing table.

TABLE 6 slice_qp_delta specifies the initial value of Qp_(Y) to be usedfor the coding blocks in the slice until modified by the value ofCuQpDeltaVal in the coding unit layer. The initial value of the Qp_(Y)quantization parameter for the slice. SliceQp_(Y), is derived asfollows: SliceQp_(Y) = 26 + init_qp_minus26 + slice_qp_delta The valueof SliceQp_(Y) shall be in the range of −QpBdOffset_(Y) to +63,inclusive. slice_cb_qp_offset specifies a difference to be added to thevalue of pps_cb_qp_offset when determining the value of the Qp′_(Cb),quantization parameter. The value of slice_cb_qp_offset shall be in therange of −12 to +12, inclusive. When slice_cb_qp_offset is not present,it is inferred to be equal to 0. The value of pps_cb_qp_offset +slice_cb_qp_offset shall be in the range of −12 to +12, inclusive.slice_cr_qp_offset specifies a difference to be added to the value ofpps_cr_qp_offset when determining the value of the Qp′_(Cr) quantizationparameter. The value of slice cr_qp_offset shall be in the range of −12to +12, inclusive. When slice_cr_qp_offset is not present, it isinferred to be equal to 0. The value of pps_cr_qp_offset +slice_cr_qp_offset shall be in the range of −12 to +12, inclusive. slicejoint_cbcr_qp_offset specifies a difference to be added to the value ofpps_joint cbcr_qp_offset when determining the value of the Qp′_(CbCr).The value of slice_joint_cbcr_qp_offset shall be in the range of −12 to+12, inclusive. When slice joint_cbcr_qp_offset is not present, it isinferred to be equal to 0. The value of pps_joint_cbcr_qp_offset +slice_joint_cbcr_qp_offset shall be in the range of −12 to +12,inclusive. The derivation process for the luma and chroma quantizationparameters begin with the inputs to the process being a luma location,variables specifying the width and height of the current coding blockand a variable specifying whether it is a single or a dual tree. Theluma, chroma and the joint chroma quantization parameters are dentotedas follows: Qp′_(Y) Qp′_(Cb) Qp′_(Cr) and Qp′_(CbCr).cu_qp_delta_sign_flag specifies the sign of CuQpDeltaVal as follows:  IfCu_qp_delta_sign_flag is equal to 0, the corresponding CuQpDeltaVal hasa positive  value.  Otherwise (cu_qp_delta_sign_flag is equal to 1), thecorresponding CuQpDeltaVal has  a negative value. Whencu_qp_delta_sign_flag is not present, it is inferred to be equal to 0.When eu_qp_delta_abs is present, the variables IsedQpDeltaCoded andCuQpDeltaVal are derived as follows:   IsCuQpDeltaCoded = 1  CuQpDeltaVal = cu_qp_delta_abs * ( 1 − 2 * cu_qp_delta_sign_flag ) Thevalue of CuQpDeltaVal shall be in the range of −( 32 + QpBdOffsetY / 2 )to +( 31 + QpBdOffsetY / 2 ), inclusive.

For example, slice_qp_delta may represent an initial value of Qp_(Y) tobe used in a coding block within a slice until it is modified by a valueof CuQpDeltaVal in a coding unit layer. For example, an initial value ofQp_(Y) for a slice, SliceQp_(Y), may be derived as26+init_qp_minus26+slice_qp_delta. A value of SliceQp_(Y) may be in arange of −QpBdOffset_(Y) to +63.

Furthermore, for example, slice_cb_qp_offset may represent a differenceto be added to a value of pps_cb_qp_offset when a value of thequantization parameter Qp′_(Cb) is determined. A value ofslice_cb_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cb_qp_offset is not present, the slice_cb_qp_offsetmay be inferred as 0. A value of pps_cb_qp_offset+slice_cb_qp_offset maybe in a range of 12 to +12.

Furthermore, for example, slice_cr_qp_offset may represent a differenceto be added to a value of pps_cr_qp_offset when a value of aquantization parameter Qp′_(Cr) is determined. A value ofslice_cr_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cr_qp_offset is not present, the slice_cr_qp_offsetmay be inferred as 0. A value of pps_cr_qp_offset+slice_cr_qp_offset maybe in a range of 12 to +12.

Furthermore, for example, slice_cbcr_qp_offset may represent adifference to be added to a value of pps_cbcr_qp_offset when a value ofa quantization parameter Qp′_(CbCr) is determined. A value ofslice_cbcr_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cbcr_qp_offset is not present, theslice_cbcr_qp_offset may be inferred as 0. A value ofpps_cbcr_qp_offset+slice_cbcr_qp_offset may be in a range of 12 to +12.

A derivation process for luma and chroma quantization parameters may bestarted based on the fact that an input for the process is a lumalocation, a parameter to designate the width and height of a currentcoding block, and a parameter to designate a single tree or a dual tree.Meanwhile, as in the aforementioned contents, a luma quantizationparameter, a chroma quantization parameter and a joint chromaquantization parameter may be represented as Qp′_(Y), Qp′_(Cb), Qp′_(Cr)and Qp′_(CbCr).

Meanwhile, for example, the syntax element cu_qp_delta_sign_flagrepresenting a sign of CuQpDeltaVal may be parsed. For example, thecu_qp_delta_sign_flag may represent the sign of CuQpDeltaVal as follows.

For example, when the cu_qp_delta_sign_flag is 0, CuQpDeltaValcorresponding to the cu_qp_delta_sign_flag may have a positive value.Alternatively, for example, when the cu_qp_delta_sign_flag is 1,CuQpDeltaVal corresponding to the cu_qp_delta_sign_flag may have anegative value. Furthermore, if the cu_qp_delta_sign_flag is notpresent, the cu_qp_delta_sign_flag may be inferred as 0.

Furthermore, for example, if cu_qp_delta_abs is present, a parameterIsCuQpDeltaCoded may be derived as 1. A parameter CuQpDeltaVal may bederived as cu_qp_delta_abs*(1-2*cu_qp_delta_sign_flag). The CuQpDeltaValmay be in a range of −(32+QpBdOffsetY/2) to +(31+QpBdOffsetY/2).

Thereafter, for example, the luma quantization parameter Qp′_(Y) may bederived as in the following equation.

$\begin{matrix}{{Qp_{Y}} = {( {( {{qP}_{Y\_{PRED}} + {CuQpDeltaVal} + 64 + {2^{*}{QpBdOffset}_{Y}}} )\%( {64 + {QpBdOffset}_{Y}} )} ) - {QpBdOffset}_{Y}}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Furthermore, if ChromaArrayType is not 0 and, treeType is SINGLE_TREE orDUAL_TREE_CHROMA, the following may be applied.

-   -   When treeType is equal to DUAL_TREE_CHROMA, a parameter Qp_(Y)        may be set identically with a luma quantization parameter Qp_(Y)        of a luma coding unit including a luma location (xCb+cbWidth/2,        yCb+cbHeight/2).    -   Parameters qP_(Cb), qP_(Cr) and qP_(CbCr) may be derived as        follows.

$\begin{matrix}{{{{qPi_{Cb}} = {Clip3( {{- {QpBdOffset}_{C}},69,\ {{Qp_{Y}} + {{pps\_ cb}{\_ qp}{\_ offset}} + {{slice\_ cb}{\_ qp}{\_ offset}}}} )}}{{qP}i_{Cr}} = {{Clip}\; 3( {{- {QpBdOffset}_{C}},69,{{Qp}_{Y} + {{pps\_ cr}{\_ qp}{\_ offset}} + {{slice\_ cr}{\_ qp}{\_ offset}}}} )}}{{qPi}_{CbCr} = {{Clip}\; 3( {{- {QpBdOffset}_{C}},69,{{Qp}_{Y} + {{pps\_ joint}{\_ cbcr}{\_ qp}{\_ offset}} + {{slice\_ joint}{\_ cbcr}{\_ qp}{\_ offset}}}} )}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

For example, when ChromaArrayType is 1, parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be set identically with a QpC value designated in Table 7based on the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

TABLE 7 qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 Qp_(C)=qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 = qPi − 6

Alternatively, when ChromaArrayType is not 1, the parameters qP_(Cb),qP_(Cr), and qP_(CbCr) may be set identically with Min (qPi, 63) basedon the same indices qPi as qPi_(Cb), qPi_(Cr), and qPi_(Cb)c,respectively.

-   -   Chroma quantization parameters for a Cb component and a Cr        component, Qp′_(Cb) and Qp′_(Cr), and a chroma quantization        parameter Qp′_(CbCr) for joint Cb-Cr coding may be derived as        follows.

$\begin{matrix}{{{Q\;{p^{\prime}}_{Cb}} = {{qP_{Cb}} + {{QpB}\;{dOffset}_{C}}}}{{{Qp}^{\prime}}_{Cr} = {{qP_{Cr}} + {QpBdOffset}_{C}}}{{Q\;{p^{\prime}}_{CbCr}} = {{qP_{CbCr}} + {QpBdOffset}_{C}}}} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

Meanwhile, this document proposes schemes for improving codingefficiency in a quantization/dequantization process.

In an embodiment, this document proposes a method of defining and usinga user defined chroma quantization mapping table, not a method ofobtaining a chroma quantization parameter value from a luma quantizationparameter value through a chroma quantization mapping table predefinedin the existing VVC Draft5 v.7 when ChromaArrayType is not 0 (e.g., whenChromaArrayType is 1). In the VVC specification text (e.g., VVC Draft5v.7), when qPi (a luma quantization parameter value) is given, Qpc(chroma quantization parameter value) is derived through the predefinedchroma quantization table (e.g., Table 7), but this document proposes amethod of deriving Qpc from qPi based on a chroma quantization mappingtable newly defined by a user. According to an embodiment of thisdocument, there is proposed a method in which a Qpc value may be derivedthrough a function relation of a qPi value, a function may be signaledas a syntax, such as an APS, an SPS or a PPS, through a user definedfunctionality method, which includes the function relation transmitvalues of predefined syntax elements, and a user defines chromaquantization table mapping based on the transmitted values. For example,since a Qpc value can be derived through the function relation of theqPi value, if syntax element values representing a correspondingfunction are transmitted, a user defined chroma quantization mappingtable may be derived in a form, such as Table 7.

In an embodiment, there is proposed a scheme for signaling informationabout syntax elements (Qpc_data) representing chroma quantizationmapping-related function as in the following table to be described laterin an adaptation parameter set (APS).

TABLE 8 adaptation_parameter_set_rbsp( ) { Descriptor adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type = = ALF_APS )   alf_data(adaptation_parameter_set_id ) else if( aps_params_type = = LMCS_APS )   lmcs_data( )  else if(aps_params_typc = = Qpc_APS ) // 2   Qpc_data( )  aps_extension_flagu(1)  if( aps_extension_flag )   while( more_rbsp_data( ) )   aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

Referring to Table 8, if the aps_params_type represents Qpc_APS, forexample, when a value of the aps_params_type is 2, Qpc_data( ) may besignaled.

Semantics for syntax elements in Table 8 may be the same as thefollowing table.

TABLE 9 adaptation_parameter_set_id provides an identifier for the APSfor reference by other syntax elements.  NOTE-APSs can be shared acrosspictures and can be different in different slices  within a picture.aps_extension_flag equal to 0 specifies that no aps_extension_data flagsyntax elements are present in the APS RBSP syntax structure.aps_extension_flag equal to 1 specifies that there areaps_extension_data_flag syntax elements present in the APS RBSP syntaxstructure. aps_extension_data_flag may have any value. Its presence andvalue do not affect decoder conformance to profiles specified in thisversion of this Specification. Decoders conforming to this version ofthis Specification shall ignore all aps_extension_data_flag syntaxelements. aps_params_type specifies the type of APS parameters carriedin the APS as specified in the Table 2 shown below.

For example, the syntax element adaptation_parameter_set_id may providean identifier of an APS referred by other syntax elements.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether aps_extension_data_flag syntax elements are present inan APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in the APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in the APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, a decodingapparatus that follows a version of this standard may ignore all syntaxelements aps_extension_data_flag.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

TABLE 10 Name of Type of aps_params_type aps_params_type APS parameters0 ALF_APS ALF parameters 1 LMCS_APS LMCS parameters 2 QP_(C)_APS Qpcdata parameters 3..7 Reserved Reserved

For example, referring to Table 10, when a value of the syntax elementaps_params_type is 0, the syntax element aps_params_type may representthat the type of APS parameters is ALF parameters. When a value of thesyntax element aps_params_type is 1, the syntax element aps_params_typemay represent that the type of APS parameters is LMCS parameters. When avalue of the syntax element aps_params_type is 2, the syntax elementaps_params_type may represent that the type of APS parameters is Qpcdata parameters. The Qpc data parameter may represent a chromaquantization data parameter.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a scheme for signaling userdefined Qp_(C) data in a picture parameter set (PPS). As an example forperforming a scheme proposed in the present embodiment, a flagrepresenting whether a PPS includes user defined data in an SPS may beintroduced. That is, a flag representing whether a PPS includes userdefined data in an SPS, may be signaled. Furthermore, according to thepresent embodiment, user defined data may be signaled in a PPS.Alternatively, user defined data may be signaled in a slice headerand/or another header set.

The flag representing whether a PPS includes user defined data may besignaled as in the following table.

TABLE 11   seq_parameter_set_rbsp( ) { Descriptor   sps_decoding_parameter_set_id u(4)   ...   Qpc_data_default_flag u(1)  ...    rbsp_trailing_bits( )   }

For example, the syntax element Qpc_data_default_flag may be a syntaxelement of the aforementioned flag. The syntax elementQpc_data_default_flag may represent whether Qpc_data( ) parameters arepresent in a PPS RBSP syntax structure. For example,Qpc_data_default_flag of 0 may represent that Qpc_data( ) parameters arenot present in the PPS RBSP syntax structure and a default table is usedto help a determination of chroma quantization. In this case, thedefault table may be the same as Table 7. Furthermore, for example,Qpc_data_default_flag of 1 may represent that Qpc_data( ) parameters maybe present in the PPS RBSP syntax structure.

Furthermore, user defined data signaled in a PPS according to thepresent embodiment may be the same as the following table.

TABLE 12   pic_parameter_set_rbsp( ) { Descriptor   pps_pic_parameter_set_id ue(v)   ...    if(Qpc_data_default_flag)    Qpc_data( )   ...    rbsp_trailing_bits( )   }

Meanwhile, for example, Qpc_data( ) may include information necessaryfor chroma quantization derivation when ChromaArrayType is 1.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a flexible structure forchroma quantization parameter (QP) derivation and combined chroma QPderivation. The present embodiment proposes a scheme for signaling aninitial flag representing whether a user defined mode in whichparameters representing a function used to derive a chroma quantizationparameter (QP) in an SPS and/or a PPS may be used is present.

For example, flag information signaled in a high level syntax proposedin the present embodiment may be the same as a table to be describedlater.

TABLE 13   high_level_syntax_parameter_set { Descriptor   ...  Qpc_data_present_flag u(1) ...   }

For example, Qpc_data_present_flag may represent whether parameters forderiving a chroma quantization parameter are present in a high levelsyntax RBSP syntax structure. For example, Qpc_data_present_flag havinga value of 0 may represent that chroma quantization parameters are notpresent in a high level syntax RBSP syntax structure. Furthermore, forexample, Qpc_data_present_flag having a value of 1 may represent thatchroma quantization parameters are present in a high level syntax RBSPsyntax structure.

Alternatively, the syntax element Qpc_data_present_flag may be used toindicate a scheme of using chroma quantization derivation in abitstream. For example, the syntax element Qpc_data_present_flag mayrepresent a tool used for chroma quantization derivation or the use of auser defined mode as follows.

For example, Qpc_data_present_flag may represent whether user definedchroma quantization is used in a bitstream. For example,Qpc_data_present_flag having a value of 0 may represent that userdefined chroma quantization is not used in a bitstream. Furthermore, forexample, Qpc_data_present_flag having a value of 1 may represent thatuser defined chroma quantization is used solely or along with anotherflag.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes an embodiment in which howchroma quantization parameters (QP), that is, Qp′_(Cb), Qp′_(Cr) andQp′_(CbCr) can be derived using user defined information signaled in onefunction. For example, according to the present embodiment, datarepresenting a function for deriving a chroma quantization parameter(QP) may be signaled, and chroma quantization parameters may be derivedbased on chroma quantization data. Data (or user defined QP mappingtable) for chroma quantization parameter derivation may be signaled asin the following table.

TABLE 14   QP_(C)_data( ) { Descriptor    qPi_min_idx ue(v)   qPi_delta_max_idx ue(v)    for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )    QP_(C)_qPi_val[ i ] ue(v)    QpOffset_(C) ue(v)   }

Semantics for syntax elements in Table 14 may be the same as thefollowing table.

TABLE 15 qPi_min_idx specifies the minimum qPi index used in the chromaquantization. qPi_delta_max_idx specifies the delta value between theQp_(C)_min_idx and the maximum qPi index used for the chroma Qp_(C)derivation. The value of qPiMaxIdx shall be greater than or equal toqPi_min_idx. The maximum index qPiMaxidx used in Qp_(C) derivation isderived as follows:   qPiMaxIdx = qPi_min_idx + qPi_delta_max_idxQp_(C)_qPi_val[i] specifies the Qp_(C) value for the i^(th) index.QpOffset_(C) specifies the offset value to be used the derivation of theQp_(C). The variable Qp_(C)Idx[ qPi ] for qPi with qPi = 0 . . .qPiMaxIdx, is derived as follows:  For qPi < qPi_min_idx, Qp_(C)Idx[qPi] is set equal to qPi.  For qPi = qPi_min_idx . . . qPiMaxIdx, thefollowing applies:    Qp_(C)Idx[ qPi ] = Qp_(C)_qPi_val[ qPi ]  ForqPi > qPiMaxIdx, Qp_(C)Idx[ qPi ] = qPi − QpOffset_(C)  The value ofQp_(C) is derived as Qp_(C)Idx[qPi].

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, a maximum index qPiMaxIdx used inQp_(c) derivation may be derived as in the following equation.

$\begin{matrix}{{qPiMaxIdx} = {{{qPi\_ min}{\_ idx}} + {{qPi\_ delta}{\_ max}{\_ idx}}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

Furthermore, for example, the syntax element Qp_(C)_qPi_val[i] mayrepresent a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used for the derivation of Qp_(C).

Furthermore, for example, a parameter Qp_(C)Idx[qPi] for qPi may bederived as follows. In this case, the qPi may be 0 to qPiMaxIdx.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        identically with Qp_(C)_qPi_val[qPi].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−QpOffset_(C).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

For example, according to the present embodiment, if a process ofderiving a quantization parameter is described in a standard format, theprocess may be represented as in the following table.

TABLE 16 Derivation process for quantization parameters Inputs to thisprocess are: a luma location ( xCb, yCb ) specifying the top-left lumasample of the current coding block relative to the top-left luma sampleof the current picture, a variable cbWidth specifying the width of thecurrent coding block in luma samples, a variable cbHeight specifying theheight of the current coding block in luma samples, a variable treeTypespecifying whether a single tree (SINGLE_TREE) or a dual tree is used topartition the CTUs and, when a dual tree is used, whether the luma(DUAL_TREE_LUMA) or chroma components (DUAL_TREE_CHROMA) are currentlyprocessed. In this process, the luma quantization parameter Qp′_(Y) andthe chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) are derived.The luma location ( xQg, yQg ), specifies the top-left luma sample ofthe current quantization group relative to the top left luma sample ofthe current picture. The horizontal and vertical positions xQg and yQgare set equal to CuQgTopLeftX and CuQgTopLeftY, respectively. NOTE-: Thecurrent quantization group is a rectangluar region inside a coding treeblock that shares the same qP_(Y)_PRED. Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y)_PRED is derived by thefollowing ordered steps:  1.The variable qP_(Y)_PREV is derived asfollows:   If one or more of the following conditions are true,qP_(Y)_PREV is set equal to   SliceQp_(Y):     The current quantizationgroup is the first quantization group in a slice.     The currentquantization group is the first quantization group in a brick.   Otherwise, qP_(Y)_PREV is set  2. The availability derivation processfor a block as specified in clause 6.4.X [Ed. (BB):   Neighbouringblocks availability checking process tbd] is invoked with the location  ( xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouringlocation   ( xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, andthe output is assigned to   availableA. The variable qP_(Y)_A is derivedas follows:    If one or more of the following conditions are true,qP_(Y)_A is set equal to qP_(Y)_PREV:     availableA is equal to FALSE.    the CTB address ctbAddrA of the CTB containing the luma coding block    covering the luma location ( xQg − 1, yQg ) is not equal toCtbAddrInBs,     where ctbAddrA is derived as follows:     xTmp = ( xQg− 1) >> MinTbLog2SizeY     yTmp = yQg >> MinTbLog2SizeY     minTbAddrA =MinTbAddrZs[ xTmp ][ yTmp ]     ctbAddrA = minTbAddrA >> ( 2 * (CtbLog2SizeY − MinTbLog2SizeY ) )    Otherwise, qP_(Y)_A is set equal tothe luma quantization parameter Qp_(Y) of the    coding unit containingthe luma coding block covering ( xQg − 1, yQg).  3. The availabilityderivation process for a block as specified in clause 6.4.X [Ed. (BB):  Neighbouring blocks availability checking process tbd] is invoked withthe location   ( xCurr, vCurr ) set equal to ( xCb, yCb ) and theneighbouring location   ( xNbY, yNbY ) set equal to (xQg, yQg − 1 ) asinputs, and the output is assigned to   availableB. The variableqP_(Y)_B is derived as follows:    If one or more of the followingconditions are true, qP_(Y)_B is set equal to qP_(Y)_PREV:    availableB is equal to FALSE.     the CTB address ctbAddrB of theCTB containing the luma coding block     covering the luma location(xQg, yQg − 1 ) is not equal to CtbAddrInBs,     where ctbAddrB isderived as follows:     xTmp = xQg >> MinTbLog2SizeY     yTmp = ( yQg −1 ) >> MinTbLog2SizeY     minTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ]    ctbAddrB =     minTbAddrB >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY )) (8-922)    Otherwise, qP_(Y)_B is set equal to the luma quantizationparameter Qp_(Y) of the    coding unit containing the luma coding blockcovering ( xQg, yQg − 1 ).  4. The predicted luma quantization parameterqP_(Y)_PRED is derived as follows:    If all the following conditionsare true, then qP_(Y)_PRED is set equal to the luma    quantizationparameter QP_(Y) of the coding unit containing the luma coding block   covering ( xQg, yQg − 1 ):     availableB is equal to TRUE.     thecurrent quantization group is the first quantization group in a CTB row    within a brick    Otherwise, qP_(Y)_PRED is derived as follows:    qP_(Y)_PRED = ( qP_(Y)_A + qP_(Y)_B + 1) >> 1 The variable Qp_(Y) isderived as follows:   QP_(Y) =   ( ( qP_(Y)_PRED + CuQpDcltaVal + 64 +2 * QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y) ) ) − Qp   BdOffset_(Y) Theluma quantization parameter Qp′_(Y) is derived as follows:   Qp′_(Y) =Qp_(Y) + QpBdOffset_(Y) When ChromaArrayType is not equal to 0 andtreeType is equal to SINGLE_TREE or DUA_TREE_CHROMA, the followingapplies:  When treeType is equal to DUAL_TREE_CHROMA, the variableQp_(Y) is set equal to the  luma quantization parameter Qp_(Y) of theluma coding unit that covers the luma location  ( xCb + cbWidth / 2,yCb + cbHeight / 2),  The variables qP_(Cb), qP_(Cr) and qP_(CbCr) arederived as follows:    qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, QpY +pps_cb_qp_offset + slice_cb_qp_offset )    qPi_(Cr) = Clip3(−QpBdOffset_(C), 69, Qpy + pps_cr_qp_offset + slice_cr_qp_offset )   qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qpy +pps_joint_cbcr_qp_offset + slice_joint_c    bcr_qp_offset )   IfChromaArrayType is equal to 1, the variables a qP_(Cb), qP_(Cr) andqP_(CbCr) are   set equal to the value of QP_(C) as specified in-Table8-15 based on the index qPi equal to   qPi_(Cb), qPi_(Cr), andaPi_(CbCr) respectively clause 7.x.x  Otherwise, the variables qP_(Cb),qP_(Cr) and qP_(CbCr) are set equal to Min( qPi, 63 ), based on the index qPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. The chroma quantization parameters for the Cb and Cr components,Qp′_(Cb) and Qp′_(Cr), and  joint Cb-Cr coding Qp′_(CbCr) are derived asfollows:   QP′_(Cb) = qP_(Cb) + QpBdOffset_(C)   Qp′_(Cr) = qP_(Cr) +QpBdOffset_(C)   Qp′_(CbCr) = qP_(CbCr) + QpBdOffset_(C)

Referring to Table 16, a derivation process for luma and chromaquantization parameter may be started based on the fact that an inputfor the process is a luma location (xCb, yCb), a parameter cbWidth andcbHeight designating the width and height of a current coding block, anda parameter treeType designating a single tree or a dual tree.Meanwhile, as in the aforementioned contents, a luma quantizationparameter and a chroma quantization parameter may be represented asQp′_(Y), Qp′_(Cb) and Qp′_(Cr).

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes an example in which a flagwithin as an SPS has a user defined mode or a default mode, syntaxelements which may be used to control the derivation of a quantizationparameter are used. An example of a syntax element which may be used toderive a quantization parameter may be the same as the following tables.Meanwhile, a structure of the syntax elements is not limited to astructure illustrated in the following tables, for example.

TABLE 17   pic_parameter set_rbsp( ) { Descriptor   pps_pic_parameter_set_id ue(v)   ...    Qpc_data_default_flag u(1)  ...

TABLE 18   slice_header( ) { Descriptor    slice_pic_parameter_set_idue(v)   ...    if Qpc_data_default_flag )     slice_Qp_(C)_aps_id u(5)   byte_alignment( )   }

TABLE 19 adaptation_parameter_set_rbsp( ) { Descriptor adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type = = ALF_APS)   alf_data(adaptation_parameter_set_id ) else if( aps_params_type = = LMCS_APS )   lmcs_data( )  else if (aps_params_type = = Qpc_APS) // 2   Qp_(C)_data( )  aps_extension_flagu(1)  if( aps_extension_flag )   while( more_rbsp_data( ) )   aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

For example, the syntax element Qpc_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qpc_data_default_flag having a value of 0 mayrepresent that the user defined mode is used for the derivation of aquantization parameter. Furthermore, for example, Qpc_data_default_flaghaving a value of 1 may represent that a default table is used for thederivation of a chroma quantization parameter. In this case, the defaulttable may be the same as Table 7. Furthermore, if the syntax elementQpc_data_default_flag is not present, the syntax elementQpc_data_default_flag may be inferred as 1.

Meanwhile, if the user defined mode is used, a corresponding sliceheader, tile group/header, or another proper header may be used tosignal an APS ID. For example, as in Table 18, a syntax elementrepresenting an APS ID through a slice header may be signaled.

For example, the syntax element slice_Qp_(c)_aps_id may representadaptation_parameter_set_id of a Qp_(c) APS referred to by a slice.TemporalId of a Qp_(c) APS NAL unit having adaptation_parameter_set_id,such as slice_Qp_(c)_aps_id, may be smaller than or equal to TemporalIdof a coded slice NAL unit. If a plurality of Qp_(c) APSs havingadaptation_parameter_set_id having the same value is referred to by twoor more slices of the same picture, a plurality of Qp_(c) APSs havingadaptation_parameter_set_id having the same value may have the samecontent.

Furthermore, an APS structure that transfers chroma quantization data,which is proposed in the present embodiment, may be the same as Table19.

For example, the syntax element adaptation_parameter_set_id may providean identifier of an APS referred to by other syntax elements.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether aps_extension_data_flag syntax elements are present inan APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in an APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in an APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, thedecoding apparatus following a version of this standard may ignore allof the syntax elements aps_extension_data_flag.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

Qpc_data( ) disclosed in Table 19 may be signaled as in the followingtable.

TABLE 20 Qp_(C)_data ( ) { Descriptor  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  Qp_(C)_prec_minus1 ue(v)  Qp_(C)_init_value(v)  for ( i = qPi_min_idx + 1 i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)  QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, a maximum index qPiMaxIdx used inQp_(c) derivation may be derived like Equation 4.

Furthermore, for example, a value obtained by adding 1 to the syntaxelement Qp_(c)_prec_minus1 may represent the number of bits used for therepresentation of the syntax lmes_delta_abs_cw[i]. A value ofQp_(c)_prec_minus1 may be in a range of 0 to BitDepth_(Y)−2.

Furthermore, for example, the syntax element Qp_(c)_init_val mayrepresent a Qp_(C) value corresponding to qPi_min_idx.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used for the derivation of Qp_(c).

For example, a parameter Qp_(C)Idx[qPi] for qPi may be derived asfollows. In this case, the qPi may be 0 to qPiMaxIdx.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(c)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−QpOffset_(C).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

As in the aforementioned embodiment, a chroma quantization parameter,that is, Qp′Cb, Qp′Cr, and Qp′CbCr, may be derived using signaled userdefined information or a default value illustrated in a default table,such as Table 7.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 21 Derivation process for quantization parameters Inputs to thisprocess are: a luma location ( xCb, yCb ) specifying the top-left lumasample of the current coding block relative to the top-left luma sampleof the current picture, a variable cbWidth specifying the width of thecurrent coding block in luma samples, a variable cbHeight specifying theheight of the current coding block in luma samples, a variable treeTypespecifying whether a single tree (SINGLE_TREE) or a dual tree is used topartition the CTUs and, when a dual tree is used, whether the luma(DUAL_TREE_LUMA) or chroma components (DUAL_TREE_CHROMA) are currentlyprocessed. In this process, the luma quantization parameter Qp′_(Y) andthe chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) are derived.The luma location ( xQg, yQg ), specifies the top-left luma sample ofthe current quantization group relative to the top left luma sample ofthe current picture. The horizontal and vertical positions xQg and yQgare set equal to CuQgTopLeftX and CuQgTopLeftY, respectively. NOTE-: Thecurrent quantization group is a rectangluar region inside a coding treeblock that shares the same qP_(Y)_PRED. Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y)_PRED is derived by thefollowing ordered steps:  1.The variable qP_(Y)_PREV is derived asfollows:   If one or more of the following conditions are true,qP_(Y)_PREV is set equal to   SliceQp_(Y):     The current quantizationgroup is the first quantization group in a slice.     The currentquantization group is the first quantization group in a brick.  Otherwise, qP_(Y)_PREV is set  2. The availability derivation processfor a block as specified in clause 6.4.X [Ed. (BB):   Neighbouringblocks availability checking process tbd] is invoked with the location  ( xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouringlocation   ( xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, andthe output is assigned to   availableA. The variable qP_(Y)_A is derivedas follows:    If one or more of the following conditions are true,qP_(Y)_A is set equal to qP_(Y)_PREV:     availableA is equal to FALSE.    the CTB address ctbAddrA of the CTB containing the luma coding block    covering the luma location ( xQg − 1, yQg ) is not equal toCtbAddrInBs,     where ctbAddrA is derived as follows:     xTmp = ( xQg− 1 ) >> MinTbLog2SizeY     yTmp = yQg >> MinTbLog2SizeY     minTbAddrA= MinTbAddrZs[ xTmp ][ yTmp ]     ctbAddrA = minTbAddrA >> ( 2 * (CtbLog2 SizeY − MinTbLog2SizeY ) )    Otherwise, qP_(Y)_A is set equalto the luma quantization parameter Qp_(Y) of the    coding unitcontaining the luma coding block covering ( xQg − 1, yQg ).  3. Theavailability derivation process for a block as specified in clause 6.4.X[Ed. (BB):   Neighbouring blocks availability checking process tbd] isinvoked with the location   ( xCurr, yCurr ) set equal to ( xCb, yCb )and the neighbouring location   ( xNbY, yNbY ) set equal to ( xQg, yQg− 1) as inputs, and the output is assigned to   availableB. The variableqP_(Y)_B is derived as follows:    If one or more of the followingconditions are true, qP_(Y)_B is Set equal to qP_(Y)_PREV:    availableB is equal to FALSE.     the CTB address ctbAddrB of theCTB containing the luma coding block     covering the luma location (xQg, yQg − 1 ) is not equal to CtbAddrInBs,     where ctbAddrB isderived as follows:     xTmp = xQg >> MinTbLog2SizeY     yTmp = ( yQg− 1) >> MinTbLog2SizeY     minTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ]    ctbAddrB =     minTbAddrB >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY )) (8-922)    Otherwise, qP_(Y)_B is set equal to the luma quantizationparameter Qp_(Y) of the    coding unit containing the luma coding blockcovering ( xQg, yQg − 1 ).  4. The predicted luma quantization parameterqP_(Y)_PRED is derived as follows:    If all the following conditionsare true, then qP_(Y)_PRED is set equal to the luma    quantizationparameter Qp_(Y) of the coding unit containing the luma coding block   covering ( xQg, yQg − 1 ):     availableB is equal to TRUE.     thecurrent quantization group is the first quantization group in a CTB row    within a brick    Otherwise, qP_(Y)_PRED is derived as follows:    qP_(Y)_PRED = ( qP_(Y)_A + qP_(Y)_B + 1) >> 1 The variable Qp_(Y) isderived as follows:   QP_(Y) =   ( ( qP_(Y)_PRED + CuQpDeltaVal + 64 +2 * QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y) ) ) − Qp   BdOffset_(Y) Theluma quantization parameter Qp′_(Y) is derived as follows:   Qp′_(Y) =Qp_(Y) + QpBdOffset_(Y) When ChromaArrayType is not equal to 0 andtreeType is equal to SINGLE_TREE or DUAL_TREE_CHROMA, the followingapplies:  When treeType is equal to DUAL_TREE_CHROMA, the variableQp_(Y) is set equal to the  luma quantization parameter Qp_(Y) of theluma coding unit that covers the luma location  ( xCb + cbWidth / 2,yCb + cbHeight / 2 ).  The variables qP_(Cb), qP_(Cr) and qP_(CbCr) arederived as follows:    qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_cb_qp_offset + slice_cb_qp_offset )    qPi_(Cr) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_cr_qp_offset + slice_cr_qp_offset )   qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_joint_cbcr_qp_offset + slice_joint_c    bcr_qp_offset )   IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toFALSE,   the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal tothe value of Qp_(C) as specified in   clause 7.x.x   If ChromaArrayTypeis equal to 2, and Qpc_data_default_flag is equal to TRUE,   thevariables qP_(Cb), qP_(Cr) and qP_(CbCr) are specified in Table 8-15based on the index qPi   equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively  Otherwise, the variables qP_(Cb), qP_(Cr) and qP_(CbCr)are set equal to Min( qPi, 63 ), based on the  index qPi equal toqP_(Cb), qPi_(Cr) and and qPi_(CbCr), respectively.  The chromaquantization parameters for the Cb and Cr components, Qp′_(Cb), andQp′_(Cr) and  joint Cb-Cr coding Qp′_(CbCr) are derived as follows:  Qp′_(Cb) = qP_(Cb) + QpBdOffset_(C)   QP′_(Cr) = qP_(Cr) +QpBdOffset_(C)   QP′_(CbCr) = qP_(CbCr) + QpBdOffset_(C)

TABLE 8-15 Specification of QpC as a function of qPi for ChromaArrayTypeequal to 1 qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QpC=qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 21, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. When ChromaArrayType is 1 andQp_(c)_data_default flag indicates true (i.e., for example, whenQp_(c)_data_default_flag is 1), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived by a default table based on the same indicesqPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes syntax elements which maybe used to control the derivation of a quantization parameter byindicating a user defined mode or a default mode through the flag of anSPS. Specifically, the present embodiment proposes a scheme forsignaling the syntax elements of the following syntax structure.Meanwhile, a structure of the syntax elements is an example, and is notlimited to a structure illustrated in the following table.

TABLE 22 Qp_(C)_data ( ) { Descriptor  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx, i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)  QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, maximum index qPiMaxIdx used inQp_(c) derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used in the derivation of Qp_(c), such as theaforementioned contents.

As in the aforementioned embodiment, chroma quantization parameter, thatis, Qp′Cb, Qp′Cr and Qp′CbCr may be derived using signaled user definedinformation or a default value used in a default table, such as Table 7.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

Referring to Table 23, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. For example, when ChromaArrayType is1 and Qp_(c)_data_default flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived identically with a value of Qp_(C) based on thesame indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively, asfollows.

For example, the parameter Qp_(C)Idx[i] may be derived as follows.

-   -   When i<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When i=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[i] may be set as        Qp_(C)_qPi_delta_val[i]+Qp_(C)Idx[i−1].    -   When i>qPiMaxIdx, Qp_(C)Idx[i] may be set as qPi−QpOffset_(C).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[i].

Furthermore, referring to Table 23, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates true (i.e., for example, whenQp_(c)_data_default_flag is 1), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived by a default table based on the same indicesqPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes syntax elements for achroma quantization (Qp_(C)) derivation parameter in an adaptationparameter set (APS). For example, an APS ID may be signaled in a sliceheader. Furthermore, for example, there may be proposed a flag within apicture parameter set (PPS) that represents whether a default table isused or a table derived from information signaled in an APS is used.Furthermore, for example, if a default table is not used, an additionalcontrol scheme for supporting access to an APS including Qp_(C) data maybe added to a slice header.

Meanwhile, according to the existing video/image standard, chroma QP maybe derived from luma QP, and may be updated by an additionally signaledchroma QP offset. The existing chroma quantization parameter Qp_(C)table may be a default table, such as Table 7.

The present embodiment proposes adding a function for signaling a chromaquantization parameter Qp_(C) as a function of an index qPi. An APS maybe used to integrate signaling schemes of Qp_(C) values.

For example, an APS according to the present embodiment may be the sameas the following table.

TABLE 24 Descriptor adaptation_parameter_set_rbsp( ) { adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type == ALF_APS )  alf_data(adaptation_parameter_set_id ) else if ( aps_params_type == LMCS_APS )  lmcs_data( )  else if (aps_params_type == Qp_(C)_APS ) // 2  Qp_(C)_data( )  aps_extension_flagu(1)  if( aps_extension_flag )  while( more_rbsp_data( ) )  aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

For example, the syntax element adaptation_parameter_set_id may providean identifier of an APS referred to by other syntax elements.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether the aps_extension_data_flag syntax elements arepresent in an APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in an APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in an APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, thedecoding apparatus following a version of this standard may ignore allof the syntax elements aps_extension_data_flag.

Qp_(c)_data( ) disclosed in Table 24 may be signaled as in the followingtable.

TABLE 25 Descriptor Qp_(C)_data ( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)  Qp_(C)Offset_(C)_present_flag u(1) if(Qp_(C)Offset_(C)_present_flag)  QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. Furthermore, for example, a value ofqPi_delta_max_idx may be in a range of 0 to 63. For example, a maximumindex qPiMaxIdx used in Qp_(c) derivation may be derived like Equation4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent a difference between Qp_(C) values for an i-th index. The adifference may also be called a delta.

Furthermore, for example, the syntax elementQp_(C)Offset_(C)_present_flag may represent whether QpOffset_(C) ispresent in a bitstream. For example, Qp_(C)Offset_(C)_present_flaghaving a value of 1 may represent that QpOffset_(C) is present in abitstream. Furthermore, for example, Qp_(C)Offset_(C)_present_flaghaving a value of 0 may represent that QpOffset_(C) is not present in abitstream. When Qp_(C)Offset_(C)_present_flag is not present,Qp_(C)Offset_(C)_present_flag may be inferred as 0.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used in the derivation of Qp_(c).

For example, a parameter Qp_(C)Idx[qPi] for qPi may be derived asfollows. In this case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   If qPi>qPiMaxIdx, when Qp_(C)Offset_(C)_present flag is 1,        Qp_(C)Idx[qPi] may be set as qPi−QpOffset_(C). If        Qp_(C)Offset_(C)_present flag is not 1, that is, if        Qp_(C)Offset_(C)_present flag is 0, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a flag signaled as a PPS asin the following table.

TABLE 26 Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) . . .  Qpc_data_default_flag u(1) . . .

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for quantization parameterderivation. For example, Qp_(c)_data_default_flag having a value of 0may represent that a user defined mode is used for quantizationparameter derivation. Furthermore, for example, Qp_(c)_data_default_flaghaving a value of 1 may represent that the aforementioned default tableis used for quantization parameter derivation. The default table may bethe same as Table 7. If Qp_(c)_data_default_flag is not present,Qp_(c)_data_default_flag may be inferred as 1.

Furthermore, the present embodiment proposes a syntax element signaledas a slice header as in the following table.

TABLE 27 Descriptor slice_header( ) { ue(v)  slice_pic_parameter_set_id. . .  if(!Qpc_data_default_flag )  slice_Qp_(C)_aps_id u(5) . . . byte_alignment( ) }

For example, the syntax element slice_Qp_(c)_aps_id may representadaptation_parameter_set_id of a Qp_(c) APS referred to by a slice.TemporalId of a Qp_(c) APS NAL unit having adaptation_parameter_set_id,such as slice_Qp_(c)_aps_id, may be smaller than or the same asTemporalId of a coded slice NAL unit. If a plurality of Qp_(c) APSshaving adaptation_parameter_set_id having the same value is referred toby two or more slices of the same picture, a plurality of Qp_(c) APSshaving adaptation_parameter_set_id having the same value may have thesame content.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

Referring to Table 28, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, in the present embodiment, it is proposed that user definedderivation of chroma quantization is signaled in an SPS as follows. Forexample, the present embodiment proposes user defined chromaquantization (Qp_(C)). For example, the flag of an SPS may representwhether a default table is used for chroma quantization derivation orthe contents of the table for chroma quantization derivation is derivedin information signaled in the SPS.

For example, the present embodiment proposes a scheme for performingchroma quantization as a function of an index qPi by using syntaxelements illustrated in the following table.

TABLE 29 Qp_(C)_data( ) { Descriptor  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the flag of an SPS representing whether a default table isused for chroma quantization derivation or whether signaled informationsignaled information is used for chroma quantization derivation, whichis proposed in the present embodiment, may be the same as the followingtable.

TABLE 30 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) . . .  Qpc_data_default_flag u(1) if(!Qp_(C)_data_default_flag)  Qp_(C)_data( ) . . .

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. Furthermore, for example,Qp_(c)_data_default_flag having a value of 1 may represent that adefault table is used for the derivation of a quantization parameter.The default table may be the same as Table 7. Furthermore, ifQp_(c)_data_default_flag is not present, the Qp_(c)_data_default_flagmay be inferred as 1.

For example, according to the present embodiment, if a process ofderiving a quantization parameter is written in a standard format, theprocess may be represented as in the following table.

Referring to Table 31, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes adding a function forsignaling a chroma quantization parameter Qp_(C) as a function of anindex qPi. For example, there may be proposed a scheme for signalingsyntax elements for a user defined table for quantization parameterderivation in a PPS. Accordingly, flexibility regarding changing a userdefined table and a default table in each picture that refers to a PPScan be provided.

Syntax elements for a signaled user defined table in a PPS proposed inthe present embodiment may be the same as the following table.

TABLE 32 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ ) Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the flag of SPS representing whether a default table isused for chroma quantization derivation or whether signaled informationis used for chroma quantization derivation, which is proposed in thepresent embodiment, may be the same as the following table.

TABLE 33 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) . . .  Qpc_data_default_flag u(1) if(!Qp_(C)_data_default_flag)  Qp_(C)_data( ) . . .

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent that chroma quantization parameterdata Qp_(c)_data( ) is used. When the Qp_(c)_data_default_flag is 0, thechroma quantization parameter data Qp_(c)_data( ) may be signaled.Furthermore, for example, Qp_(c)_data_default_flag having a value of 1may represent that a default table is used for the derivation of aquantization parameter. The default table may be the same as Table 7.Furthermore, if Qp_(c)_data_default_flag is not present, theQp_(c)_data_default_flag may be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

Referring to Table 34, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example,ChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a common mode in which achroma quantization parameter Qp_(C) is derived and signaled.

Chroma quantization parameter data Qp_(c)_data( ) for a chromaquantization parameter proposed in the present embodiment may besignaled as in the following table.

TABLE 35 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx, i++ ) Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a scheme for signaling aflag representing whether a default table is used for chromaquantization derivation or whether signaled information is used forchroma quantization derivation. The flag may be signaled through a highlevel syntax, such as a sequence parameter set (SPS), or a pictureparameter set (PPS). The flag signaled through a high level syntax maybe the same as the following table.

TABLE 36 Descriptor high_level_parameter_set( ) { . . . Qpc_data_default_flag u(1)  if(!Qp_(C)_data_default_flag)  Qp_(C)_data() . . .

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent chroma quantization parameter dataQp_(c)_data) is used. When the Qp_(c)_data_default_flag is 0, the chromaquantization parameter data Qp_(c)_data( ) may be signaled. Furthermore,for example, Qp_(c)_data_default_flag having a value of 1 may representthat a default table is used for the derivation of a quantizationparameter. The default table may be the same as Table 7. Furthermore, ifQp_(c)_data_default_flag is not present, the Qp_(c)_data_default_flagmay be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

Referring to Table 37, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a scheme for deriving achroma quantization parameter Qp_(C) table without an offset. Thepresent embodiment may be proposed to be used along with an APS or to beused independently. For example, a syntax structure of APS integratedwith chroma quantization data may be the same as the following table.

TABLE 38 Descriptor Qp_(C)_data ( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMax1dx; i++ ) Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent a difference between Qp_(C) values for an i-th index. Thedifference may also be called a delta.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows. Inthis case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   qPi=When qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes, as an example, a scheme inwhich a delta (or a difference) between continuous Qp_(C) values islimited to 1.

For example, the present embodiment proposes a scheme in which userdefined chroma quantization (Qp_(C)) is additionally included in theexisting image/video standard. For example, the flag of a sequenceparameter set (SPS) proposed in the present embodiment may representwhether the existing default table is used for chroma quantizationparameter derivation or whether the contents of a table is derived basedon information signaled in an SPS. A scheme suitable for a coded imagemay be selected by accommodating user defined chroma quantizationaccording to the present embodiment, and coding efficiency can beimproved.

For example, the present embodiment proposes adding a function forsignaling chroma quantization Qp_(C) as a function of an index qPi byusing syntax elements as in the following table.

TABLE 39 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx uc(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ ) Qp_(C)_qPi_flag[ i ] u(1) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 1 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of1 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element QpC_qPi_flag[i] mayrepresent whether a Qp_(C) value is increased by 1. That is, forexample, the syntax element QpC_qPi_flag[i] may represent whether ani-th Qp_(C) value has increased by 1 compared to an (i−1)-th Qp_(C)value. For example, QpC_qPi_flag[i] having a value of 1 may represent aQp_(C) value is increased by 1. QpC_qPi_flag[i] having a value of 0 mayrepresent that a Qp_(C) value is not increased.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows. Inthis case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_flag[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a scheme for signaling aflag representing whether a default table is used for chromaquantization derivation or whether signaled information is used forchroma quantization derivation. The flag may be signaled through a highlevel syntax, such as a sequence parameter set (SPS), or a pictureparameter set (PPS). The flag signaled through a high level syntax maybe the same as the following table.

TABLE 40 Descriptor seq_parameter_set_rbsp( ) {   sps_decoding_parameter_set_id u(4) . . .    Qp_(C)_data_default_flagu(1)  if(!Qp_(C)_data_default_flag)   Qp_(C)_data( ) . . .

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent that chroma quantization parameterdata Qp_(c)_data( ) is used. When the Qp_(c)_data_default_flag is 0, thechroma quantization parameter data Qp_(c)_data( ) may be signaled.Furthermore, for example, Qp_(c)_data_default_flag having a value of 1may represent that a default table is used for the derivation of aquantization parameter. The default table may be the same as Table 7.Furthermore, if Qp_(c)_data_default_flag is not present, theQp_(c)_data_default_flag may be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

Referring to Table 41, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

FIG. 7 schematically shows an image encoding method by an encodingapparatus according to the present document. The method disclosed inFIG. 7 may be performed by the encoding apparatus disclosed in FIG. 2.Specifically, for example, S700 in FIG. 7 may be performed by thepredictor of the encoding apparatus. S710 in FIG. 7 may be performed bythe subtractor of the encoding apparatus. S720 in FIG. 7 may beperformed by the residual processor of the encoding apparatus. S730 toS740 may be performed by the entropy encoder of the encoding apparatus.Furthermore, although not illustrated, a process of generatingreconstructed samples and a reconstructed picture based on residualsamples and prediction samples may be performed by the adder of theencoding apparatus.

The encoding apparatus derives prediction samples of a current chromablock based on inter prediction or intra prediction (S700). The encodingapparatus may derive the prediction samples of the current chroma blockbased on a prediction mode. In this case, various prediction methodsdisclosed in this document, such as inter prediction or intraprediction, may be used.

For example, the encoding apparatus may determine whether to performinter prediction or intra prediction on the current chroma block, andmay determine a detailed inter prediction mode or a detailed intraprediction mode based on RD costs. The encoding apparatus may deriveprediction samples for the current chroma block based on the determinedmode.

The encoding apparatus derives residual samples of the current chromablock based on the prediction samples (S710). For example, the encodingapparatus may derive the residual samples by subtracting originalsamples and the prediction samples for the current chroma block.

The encoding apparatus generates quantization parameter data forcombined chroma coding of the residual samples based on a chroma type ofthe current chroma block (S720). The encoding apparatus may generate thequantization parameter data for the combined chroma coding of theresidual samples based on a chroma type. In this case, the chroma typemay mean the aforementioned ChromaArrayType. For example, if a value ofthe chroma type is not 0, the encoding apparatus may generate thequantization parameter data for the combined chroma coding. For example,when a value of the chroma type is 1, the decoding apparatus maygenerate the quantization parameter data for the combined chroma coding.In this case, when a value of the chroma type is 0, the chroma type maybe a Monochrome format. When a value of the chroma type is 1, the chromatype may be a 4:2:0 format. When a value of the chroma type is 2, thechroma type may be a 4:2:2 format. When a value of the chroma type is 3,the chroma type may be a 4:4:4 format. Furthermore, the combined chromacoding may also be called joint coding of a chroma component. The chromacomponent may include a Cb component and/or a Cr component.

Furthermore, for example, the encoding apparatus may determine whetherto perform combined chroma coding on residual samples for a chromacomponent based on a chroma type (e.g., when a value of the chroma typeis not 0). If the combined chroma coding is performed on the residualsamples, quantization parameter data for the combined chroma coding ofthe residual samples may be generated. For example, the quantizationparameter data may be signaled through a high level syntax. For example,the quantization parameter data may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

For example, the quantization parameter data may include a syntaxelement representing a starting index of a chroma quantization parametertable for the combined chroma coding and/or a syntax elementrepresenting a difference between the starting index and last index ofthe chroma quantization parameter table. The syntax element representingthe starting index may be the aforementioned qPi_min_idx. Furthermore,the syntax element representing the difference between the startingindex and the last index may be qPi_delta_max_idx. Furthermore, thechroma quantization parameter table may also be called a chromaquantization parameter mapping table or a user defined quantizationparameter mapping table. Furthermore, the starting index may also becalled a minimum index. Furthermore, for example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a high level syntax. For example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a sequence parameter set (SPS), a picture parameter set(PPS), a slice header, or an adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may includesyntax elements for quantization parameter values of indices of thechroma quantization parameter table. That is, for example, thequantization parameter data may include a syntax element for aquantization parameter value of each of indices of the chromaquantization parameter table. The syntax element for the quantizationparameter value of the index may be the aforementionedQp_(C)_qPi_val[i]. Furthermore, for example, the syntax element for thequantization parameter value of the index may be signaled through a highlevel syntax. For example, the syntax element for the quantizationparameter value of the index may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may include asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding. The syntaxelement representing the offset may be the aforementioned QpOffset_(C).Furthermore, for example, the syntax element representing the offset maybe signaled through a high level syntax. For example, the syntax elementrepresenting the offset may be signaled through a sequence parameter set(SPS), a picture parameter set (PPS), a slice header, or an adaptationparameter set (APS).

Meanwhile, for example, the encoding apparatus may derive thequantization parameter for the combined chroma coding based on thequantization parameter data. The quantization parameter for the combinedchroma coding may represent the aforementioned QP′_(CbCr).

For example, as in the aforementioned contents, the chroma quantizationparameter table may be derived based on the syntax element representingthe starting index of the chroma quantization parameter table, thesyntax element representing the difference between the starting indexand last index of the chroma quantization parameter table and/or thesyntax elements for the quantization parameter values of the indices ofthe chroma quantization parameter table. That is, for example, thechroma quantization parameter table for the combined chroma coding maybe derived based on the quantization parameter data. Thereafter, anindex for the combined chroma coding may be derived based on aquantization parameter for a luma component. The quantization parameterfor the combined chroma coding may be derived based on a quantizationparameter for the index of the chroma quantization parameter table. Thatis, for example, the quantization parameter for the combined chromacoding may be derived based on a quantization parameter for the sameindex as the quantization parameter of the luma component in the chromaquantization parameter table.

For example, the quantization parameter (e.g., QP′_(CbCr)) for thecombined chroma coding may be derived by adding an offset to thequantization parameter (e.g., QP_(CbCr)) for the index of the chromaquantization parameter table. The offset may be derived based on asyntax element representing an offset for the derivation of thequantization parameter for the combined chroma coding.

The encoding apparatus generates a flag representing whether thequantization parameter data for the combined chroma coding is present(S730). For example, the encoding apparatus may generate whether a flagrepresenting whether the quantization parameter data for the combinedchroma coding is present based on the chroma type. For example, when avalue of the chroma type is not 0, the encoding apparatus may generatethe flag representing whether the quantization parameter data for thecombined chroma coding is present. For example, when a value of thechroma type is 1, the encoding apparatus may generate the flagrepresenting whether the quantization parameter data for the combinedchroma coding is present. For example, the syntax element for the flagmay be the aforementioned Qp_(C)_data_present_flag.

For example, when a value of the flag is 0, the flag may represent thatthe quantization parameter data for the combined chroma coding is notpresent. When a value of the flag is 1, the flag may represent that thequantization parameter data for the combined chroma coding is present.

Furthermore, for example, the flag may be signaled through a high levelsyntax. For example, the flag may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

The encoding apparatus encodes image information including predictioninformation and residual information for the current chroma block, thequantization parameter data and the flag (S740). The encoding apparatusmay encode the image information. The image information may include theprediction information and residual information for the current chromablock, the quantization parameter data and the flag.

For example, the encoding apparatus may generate and encode predictioninformation for the current block. The prediction information mayinclude prediction mode information representing a prediction mode ofthe current block. The image information may include the predictioninformation.

Furthermore, for example, the encoding apparatus may encode residualinformation for the residual samples. For example, the encodingapparatus may derive transform coefficients based on the residualsamples, and may generate the residual information based on thetransform coefficients. The image information may include the residualinformation. For example, the residual information may include syntaxelements for the transform coefficients of the current chroma block. Forexample, the syntax elements may include syntax elements, such ascoded_sub_block_flag, sig_coeff_flag, coeff_sign_flag,abs_level_gtl_flag, par_level_flag, abs_level_gtX_flag, abs_remainder,and/or coeff_sign_flag.

Furthermore, for example, the encoding apparatus may encode imageinformation including the quantization parameter data and the flag.Furthermore, for example, the encoding apparatus may output the imageinformation in the form of a bitstream by encoding the imageinformation.

Meanwhile, the bitstream including the image information may betransmitted to the decoding apparatus over a network or a (digital)storage medium. In this case, the network may include a broadcastingnetwork and/or a communication network. The digital storage medium mayinclude various storage media, such as a USB, an SD, a CD, a DVD,Blu-ray, an HDD, and an SSD.

FIG. 8 schematically shows an encoding apparatus for performing an imageencoding method according to this document. The method disclosed in FIG.7 may be performed by the encoding apparatus disclosed in FIG. 8.Specifically, for example, the predictor of the encoding apparatus inFIG. 8 may perform S700. The subtractor of the encoding apparatus mayperform S710. The residual processor of the encoding apparatus mayperform S720. The entropy encoder of the encoding apparatus may performS730 to S740. Furthermore, although not illustrated, a process ofgenerating reconstructed samples and a reconstructed picture based onthe residual samples and prediction samples may be performed by theadder of the encoding apparatus.

FIG. 9 schematically shows an image decoding method by a decodingapparatus according to this document. The method disclosed in FIG. 9 maybe performed by the decoding apparatus disclosed in FIG. 3.Specifically, for example, S900 to S910 in FIG. 9 may be performed bythe entropy decoder of the decoding apparatus. S920 and S940 to S960 inFIG. 9 may be performed by the residual processor of the decodingapparatus. S930 in FIG. 9 may be performed by the predictor of thedecoding apparatus. S970 in FIG. 9 may be performed by the adder of thedecoding apparatus.

The decoding apparatus obtains image information, including a flagrepresenting whether quantization parameter data for combined chromacoding is present, and prediction information and residual informationfor a current chroma block, based on a chroma type (S900). The decodingapparatus may obtain the image information through a bitstream. Forexample, the image information may include information for a chromaquantization parameter. For example, the image information may includethe flag representing whether the quantization parameter data for thecombined chroma coding is present. For example, the decoding apparatusmay obtain the flag representing whether the quantization parameter datafor the combined chroma coding is present based on the chroma type. Inthis case, the chroma type may mean the aforementioned ChromaArrayType.For example, when a value of the chroma type is not 0, the decodingapparatus may obtain a flag representing whether quantization parameterdata for combined chroma coding is present. For example, when a value ofthe chroma type is 1, the decoding apparatus may obtain a flagrepresenting whether quantization parameter data for combined chromacoding is present. In this case, when a value of the chroma type is 0,the chroma type may be a monochrome format. When a value of the chromatype is 1, the chroma type may be a 4:2:0 format. When a value of thechroma type is 2, the chroma type may be a 4:2:2 format. When a value ofthe chroma type is 3, the chroma type may be a 4:4:4 format.Furthermore, the combined chroma coding may also be called joint codingof a chroma component. The chroma component may include a Cb componentand/or a Cr component. For example, a syntax element for the flag may bethe aforementioned Qp_(C)_data_present_flag.

For example, when a value of the flag is 0, the flag may represent thatthe quantization parameter data for the combined chroma coding is notpresent. When a value of the flag is 1, the flag may represent that thequantization parameter data for the combined chroma coding is present.

Furthermore, for example, the flag may be signaled through a high levelsyntax. For example, the flag may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

Furthermore, for example, the image information may include theprediction information and/or the residual information for the currentchroma block. For example, the image information may includeprediction-related information for the current block. Theprediction-related information may include prediction mode information.The prediction mode information may represent whether inter predictionor intra prediction is applied to the current block. Furthermore, forexample, the residual information may include syntax elements fortransform coefficients of the current chroma block. For example, thesyntax elements may include syntax elements, such ascoded_sub_block_flag, sig_coeff_flag, coeff_sign_flag,abs_level_gtl_flag, par_level_flag, abs_level_gtX_flag, abs_remainderand/or coeff_sign_flag.

The decoding apparatus obtains the quantization parameter data for thecombined chroma coding based on the flag (S910). The decoding apparatusmay obtain the quantization parameter data for the combined chromacoding based on the flag. For example, the decoding apparatus may obtainthe quantization parameter data for the combined chroma coding based onthe flag representing that the quantization parameter data for thecombined chroma coding is present. That is, for example, when a value ofthe flag is 1, the decoding apparatus may obtain the quantizationparameter data for the combined chroma coding. Furthermore, for example,the quantization parameter data may be signaled through a high levelsyntax. For example, the quantization parameter data may be signaledthrough a sequence parameter set (SPS), a picture parameter set (PPS), aslice header, or an adaptation parameter set (APS).

For example, the quantization parameter data may include a syntaxelement representing a starting index of a chroma quantization parametertable for the combined chroma coding and/or a syntax elementrepresenting a difference between the starting index and last index ofthe chroma quantization parameter table. The syntax element representingthe starting index may be the aforementioned qPi_min_idx. Furthermore,the syntax element representing the difference between the startingindex and the last index may be qPi_delta_max_idx. Furthermore, thechroma quantization parameter table may also be called a chromaquantization parameter mapping table or a user defined quantizationparameter mapping table. Furthermore, the starting index may also becalled a minimum index. Furthermore, for example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a high level syntax. For example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a sequence parameter set (SPS), a picture parameter set(PPS), a slice header, or an adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may includesyntax elements for quantization parameter values of indices of thechroma quantization parameter table. That is, for example, thequantization parameter data may include a syntax element for aquantization parameter value of each of indices of the chromaquantization parameter table. The syntax element for the quantizationparameter value of the index may be the aforementionedQp_(C)_qPi_val[i]. Furthermore, for example, the syntax element for thequantization parameter value of the index may be signaled through a highlevel syntax. For example, the syntax element for the quantizationparameter value of the index may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may include asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding. The syntaxelement representing the offset may be the aforementioned QpOffset_(C).Furthermore, for example, the syntax element representing the offset maybe signaled through a high level syntax. For example, the syntax elementrepresenting the offset may be signaled through a sequence parameter set(SPS), a picture parameter set (PPS), a slice header, or an adaptationparameter set (APS).

The decoding apparatus derives a chroma quantization parameter tablebased on the quantization parameter data (S920). The decoding apparatusmay derive a chroma quantization parameter table for the combined chromacoding based on the quantization parameter data. The chroma quantizationparameter table may be called a chroma quantization parameter mappingtable or a user defined quantization parameter mapping table.

For example, as in the aforementioned contents, the chroma quantizationparameter table may be derived based on the syntax element representingthe starting index of the chroma quantization parameter table, thesyntax element representing the difference between the starting indexand last index of the chroma quantization parameter table and/or thesyntax elements for the quantization parameter values of the indices ofthe chroma quantization parameter table. That is, for example, thechroma quantization parameter table for the combined chroma coding maybe derived based on the quantization parameter data.

The decoding apparatus derives the quantization parameter for thecombined chroma coding based on the chroma quantization parameter table(S930). The quantization parameter for the combined chroma coding mayrepresent the aforementioned QP′_(CbCr).

For example, an index for the combined chroma coding may be derivedbased on a quantization parameter for a luma component. The quantizationparameter for the combined chroma coding may be derived based on aquantization parameter for the index of the chroma quantizationparameter table. That is, for example, the quantization parameter forthe combined chroma coding may be derived based on the quantizationparameter for the same index as the quantization parameter of the lumacomponent in the chroma quantization parameter table.

Furthermore, for example, the quantization parameter (e.g., QP′_(CbCr))for the combined chroma coding may be derived by adding an offset to thequantization parameter (e.g., QP_(CbCr)) for the index of the chromaquantization parameter table. The offset may be derived based on asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding.

The decoding apparatus derives prediction samples of the current chromablock based on the prediction information (S940). The decoding apparatusmay determine whether inter prediction or intra prediction is applied tothe current chroma block based on the prediction information, and mayperform prediction based on the prediction samples.

For example, the decoding apparatus may derive a prediction mode appliedto the current chroma block based on the prediction information, and mayderive prediction samples of the current chroma block based on theprediction mode. For example, if inter prediction is applied to thecurrent block, the decoding apparatus may derive motion information ofthe current chroma block based on prediction-related informationincluded in the image information, and may derive prediction samples ofthe current chroma block based on the motion information. Furthermore,for example, if intra prediction is applied to the current chroma block,the decoding apparatus may derive reference samples based on neighborsamples of the current chroma block, and may derive prediction samplesof the current chroma block based on the reference samples and an intraprediction mode of the current chroma block. The reference samples mayinclude top reference samples and left reference samples of the currentchroma block. For example, when the size of the current chroma block isN×N, an x component of a top-left sample position of the current chromablock is 0 and a y component thereof is 0, the left reference samplesmay be p[−1][0] to p[−1][2N−1], and the top reference samples may bep[0][−1] to p[2N−1][−1].

The decoding apparatus derives transform coefficients of the currentchroma block based on the residual information (S950). The decodingapparatus may derive the transform coefficients of the current chromablock based on the residual information. The residual information mayinclude coefficient level information and sign flag information for thetransform coefficients.

For example, an absolute level of the transform coefficients may bederived as a value indicated by coefficient level information includedin the residual information. The sign of the transform coefficient maybe derived as a sign indicated by the sign flag information.

The decoding apparatus derives residual samples by dequantizing thetransform coefficients based on the quantization parameter (S960). Thedecoding apparatus may derive the residual samples based on thequantization parameter. For example, the decoding apparatus may derivethe residual samples by dequantizing the transform coefficients based onthe quantization parameter. Alternatively, for example, the decodingapparatus may derive inverse-transformed transform coefficients byinverse-transforming the transform coefficients, and may derive theresidual samples by dequantizing the inverse-transformed transformcoefficients based on the quantization parameter.

The decoding apparatus generates a reconstructed picture based on theprediction samples and the residual samples (S970). For example, thedecoding apparatus may generate the reconstructed picture based on theprediction samples and the residual samples.

Meanwhile, for example, the decoding apparatus may generatereconstructed samples and the reconstructed picture through the additionof the prediction samples and the residual samples.

Thereafter, if necessary, in order to improve subjective/objectivepicture quality, an in-loop filtering procedure, such as deblockingfiltering, an SAO and/or an ALF procedure, may be applied to thereconstructed samples as described above.

FIG. 10 schematically illustrates the decoding apparatus performing theimage decoding method according to this document. The method disclosedin FIG. 9 may be performed by the decoding apparatus disclosed in FIG.10. Specifically, for example, the entropy decoder of the decodingapparatus of FIG. 10 may perform S900 to S910 in FIG. 9. The residualprocessor of the decoding apparatus of FIG. 10 may perform S920 and S940to S960 of FIG. 9. The predictor of the decoding apparatus of FIG. 10may perform S930 in FIG. 9. The adder of the decoding apparatus of FIG.10 may perform S970 in FIG. 9.

According to the present disclosure, the chroma quantization parametertable for quantization parameter derivation can be determined based on aflag representing whether to transmit quantization parameter data forquantization parameter derivation for a chroma component. Codingefficiency can be improved by performing coding based on a quantizationparameter according to characteristics of an image.

Furthermore, according to the present disclosure, a chroma quantizationparameter table for a chroma component can be determined based onsignaled chroma quantization data. Coding efficiency can be improved byperforming coding based on a quantization parameter according tocharacteristics of an image.

In the above-described embodiment, the methods are described based onthe flowchart having a series of steps or blocks. The present disclosureis not limited to the order of the above steps or blocks. Some steps orblocks may occur simultaneously or in a different order from other stepsor blocks as described above. Further, those skilled in the art willunderstand that the steps shown in the above flowchart are notexclusive, that further steps may be included, or that one or more stepsin the flowchart may be deleted without affecting the scope of thepresent disclosure.

The embodiments described in this specification may be performed bybeing implemented on a processor, a microprocessor, a controller or achip. For example, the functional units shown in each drawing may beperformed by being implemented on a computer, a processor, amicroprocessor, a controller or a chip. In this case, information forimplementation (e.g., information on instructions) or algorithm may bestored in a digital storage medium.

In addition, the decoding apparatus and the encoding apparatus to whichthe present disclosure is applied may be included in a multimediabroadcasting transmission/reception apparatus, a mobile communicationterminal, a home cinema video apparatus, a digital cinema videoapparatus, a surveillance camera, a video chatting apparatus, areal-time communication apparatus such as video communication, a mobilestreaming apparatus, a storage medium, a camcorder, a VoD serviceproviding apparatus, an Over the top (OTT) video apparatus, an Internetstreaming service providing apparatus, a three-dimensional (3D) videoapparatus, a teleconference video apparatus, a transportation userequipment (e.g., vehicle user equipment, an airplane user equipment, aship user equipment, etc.) and a medical video apparatus and may be usedto process video signals and data signals. For example, the Over the top(OTT) video apparatus may include a game console, a blue-ray player, aninternet access TV, a home theater system, a smart phone, a tablet PC, aDigital Video Recorder (DVR), and the like.

Furthermore, the processing method to which the present disclosure isapplied may be produced in the form of a program that is to be executedby a computer and may be stored in a computer-readable recording medium.Multimedia data having a data structure according to the presentdisclosure may also be stored in computer-readable recording media. Thecomputer-readable recording media include all types of storage devicesin which data readable by a computer system is stored. Thecomputer-readable recording media may include a BD, a Universal SerialBus (USB), ROM, PROM, EPROM, EEPROM, RAM, CD-ROM, a magnetic tape, afloppy disk, and an optical data storage device, for example.Furthermore, the computer-readable recording media includes mediaimplemented in the form of carrier waves (e.g., transmission through theInternet). In addition, a bit stream generated by the encoding methodmay be stored in a computer-readable recording medium or may betransmitted over wired/wireless communication networks.

In addition, the embodiments of the present disclosure may beimplemented with a computer program product according to program codes,and the program codes may be performed in a computer by the embodimentsof the present disclosure. The program codes may be stored on a carrierwhich is readable by a computer.

FIG. 11 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

The content streaming system to which the embodiment(s) of the presentdisclosure is applied may largely include an encoding server, astreaming server, a web server, a media storage, a user device, and amultimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. Into digitaldata to generate a bit stream and transmit the bit stream to thestreaming server. As another example, when the multimedia input devicessuch as smartphones, cameras, camcorders, etc. directly generate a bitstream, the encoding server may be omitted.

The bit stream may be generated by an encoding method or a bit streamgenerating method to which the embodiment(s) of the present disclosureis applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bit stream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server deliversit to a streaming server, and the streaming server transmits multimediadata to the user. In this case, the content streaming system may includea separate control server. In this case, the control server serves tocontrol a command/response between devices in the content streamingsystem.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content is received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bit stream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (e.g., Smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like. Each server in the content streaming system maybe operated as a distributed server, in which case data received fromeach server may be distributed.

The claims described in the present disclosure may be combined invarious ways. For example, the technical features of the method claimsof the present disclosure may be combined to be implemented as anapparatus, and the technical features of the apparatus claims of thepresent disclosure may be combined to be implemented as a method. Inaddition, the technical features of the method claim of the presentdisclosure and the technical features of the apparatus claim may becombined to be implemented as an apparatus, and the technical featuresof the method claim of the present disclosure and the technical featuresof the apparatus claim may be combined to be implemented as a method.

What is claimed is:
 1. An image decoding method, performed by a decodingapparatus, comprising: obtaining image information including predictioninformation and residual information for a current chroma block, and aflag representing whether quantization parameter data for combinedchroma coding is present, based on a chroma type; obtaining thequantization parameter data for the combined chroma coding based on theflag; obtaining quantization parameter data for a chroma cb componentand quantization parameter data for a chroma cr component; deriving achroma quantization parameter table for the combined chroma coding basedon the quantization parameter data for the combined chroma coding;deriving a chroma quantization parameter table for the chroma cbcomponent and a chroma quantization parameter table for the chroma crcomponent based on the quantization parameter data for the chroma cbcomponent and the quantization parameter data for the chroma crcomponent; deriving a quantization parameter for the combined chromacoding based on the chroma quantization parameter table for the combinedchroma coding; deriving a quantization parameter for the chroma cbcomponent and a quantization parameter for the chroma cr component basedon the chroma quantization parameter table for the chroma cb componentand the chroma quantization parameter table for the chroma cr component;deriving prediction samples of the current chroma block based on theprediction information; deriving transform coefficients of the currentchroma block based on the residual information; deriving residualsamples by dequantizing the transform coefficients based on aquantization parameter related to the current chroma block among thequantization parameter for the combined chroma coding, the quantizationparameter for the chroma cb component and the quantization parameter forthe chroma cr component; and generating a reconstructed picture based onthe prediction samples and the residual samples, wherein thequantization parameter data for the combined chroma coding includes asyntax element for a starting quantization parameter of the chromaquantization parameter table for the combined chroma coding and a syntaxelement for a number of quantization parameters in the chromaquantization parameter table for the combined chroma coding.
 2. Theimage decoding method of claim 1, wherein deriving the predictionsamples of the current chroma block based on the prediction informationcomprises: deriving a prediction mode of the current chroma block basedon the prediction information; and deriving the prediction samples basedon the prediction mode.
 3. The image decoding method of claim 1, whereinthe quantization parameter data for the combined chroma coding includesa syntax element representing an offset for deriving the quantizationparameter for the combined chroma coding.
 4. The image decoding methodof claim 3, wherein the flag is signaled through a picture parameter set(PPS).
 5. The image decoding method of claim 1, wherein: when a value ofthe flag is 0, the flag represents that the quantization parameter datafor the combined chroma coding is not present, and when a value of theflag is 1, the flag represents that the quantization parameter data forthe combined chroma coding is present.
 6. The image decoding method ofclaim 1, wherein the flag is signaled through a sequence parameter set(SPS).
 7. The image decoding method of claim 1, wherein the quantizationparameter data for the combined chroma coding includes syntax elementsfor quantization parameter values of indices of the chroma quantizationparameter table for the combined chroma coding.
 8. The image decodingmethod of claim 1, wherein a value of the chroma type is not
 0. 9. Theimage decoding method of claim 1, wherein the quantization parameter forthe combined chroma coding is derived based on a quantization parameterfor an index identical with an index of a quantization parameter for aluma component corresponding to the current chroma block.
 10. An imageencoding method, performed by an encoding apparatus, comprising:deriving prediction samples of a current chroma block based on interprediction or intra prediction; deriving residual samples of the currentchroma block based on the prediction samples; generating quantizationparameter data for combined chroma coding of the residual samples basedon a chroma type of the current chroma block; generating a flagrepresenting whether the quantization parameter data for the combinedchroma coding is present; generating quantization parameter data for achroma cb component and quantization parameter data for a chroma crcomponent; and encoding image information including predictioninformation and residual information for the current chroma block, thequantization parameter data for the combined chroma coding, thequantization parameter data for the chroma cb component, thequantization parameter data for the chroma cr component and the flag,wherein the quantization parameter data for the combined chroma codingincludes a syntax element for a starting quantization parameter of achroma quantization parameter table for the combined chroma coding and asyntax element for a number of quantization parameters in the chromaquantization parameter table for the combined chroma coding.
 11. Theimage encoding method of claim 10, wherein the prediction informationincludes prediction mode information representing a prediction mode ofthe current chroma block.
 12. The image encoding method of claim 10,wherein the flag is signaled through a sequence parameter set (SPS). 13.The image encoding method of claim 10, wherein the quantizationparameter data includes syntax elements for quantization parametervalues of indices of the chroma quantization parameter table.
 14. Theimage encoding method of claim 10, wherein the flag is signaled througha picture parameter set (PPS).
 15. Anon-transitory computer-readablestorage medium storing a bitstream generated by a method, the methodcomprising: deriving prediction samples of a current chroma block basedon inter prediction or intra prediction; deriving residual samples ofthe current chroma block based on the prediction samples; generatingquantization parameter data for combined chroma coding of the residualsamples based on a chroma type of the current chroma block; generating aflag representing whether the quantization parameter data for thecombined chroma coding is present; generating quantization parameterdata for a chroma cb component and quantization parameter data for achroma cr component; encoding image information including predictioninformation and residual information for the current chroma block, thequantization parameter data for the combined chroma coding, thequantization parameter data for the chroma cb component, thequantization parameter data for the chroma cr component and the flag;and generating the bitstream including the image information, whereinthe quantization parameter data for the combined chroma coding includesa syntax element for a starting quantization parameter of a chromaquantization parameter table for the combined chroma coding and a syntaxelement for a number of quantization parameters in the chromaquantization parameter table for the combined chroma coding.